diff --git a/internal/signermsgs/en_error_messges.go b/internal/signermsgs/en_error_messges.go
index 95d832c..0e5a700 100644
--- a/internal/signermsgs/en_error_messges.go
+++ b/internal/signermsgs/en_error_messges.go
@@ -104,4 +104,8 @@ var (
 	MsgInvalidEIP155TransactionV   = ffe("FF22085", "Invalid V value from EIP-155 transaction (chainId=%d)")
 	MsgInvalidChainID              = ffe("FF22086", "Invalid chainId expected=%d actual=%d")
 	MsgSigningInvalidCompactRSV    = ffe("FF22087", "Invalid signature data (compact R,S,V) length=%d (expected=65)")
+	MsgInvalidNumberString         = ffe("FF22088", "Invalid integer string '%s'")
+	MsgInvalidIntPrecisionLoss     = ffe("FF22089", "String %s cannot be converted to integer without losing precision")
+	MsgInvalidUint64PrecisionLoss  = ffe("FF22090", "String %s cannot be converted to a uint64 without losing precision")
+	MsgInvalidJSONTypeForBigInt    = ffe("FF22091", "JSON parsed '%T' cannot be converted to an integer")
 )
diff --git a/pkg/abi/inputparsing.go b/pkg/abi/inputparsing.go
index 296386f..c347b29 100644
--- a/pkg/abi/inputparsing.go
+++ b/pkg/abi/inputparsing.go
@@ -19,6 +19,7 @@ package abi
 import (
 	"context"
 	"encoding/hex"
+	"encoding/json"
 	"fmt"
 	"math/big"
 	"reflect"
@@ -26,6 +27,7 @@ import (
 
 	"github.com/hyperledger/firefly-common/pkg/i18n"
 	"github.com/hyperledger/firefly-signer/internal/signermsgs"
+	"github.com/hyperledger/firefly-signer/pkg/ethtypes"
 )
 
 var (
@@ -128,68 +130,49 @@ func getFloat64IfConvertible(v interface{}) (float64, bool) {
 // with a focus on those generated by the result of an Unmarshal using Go's default
 // unmarshalling.
 func getIntegerFromInterface(ctx context.Context, desc string, v interface{}) (*big.Int, error) {
-	i := new(big.Int)
 	switch vt := v.(type) {
+	case json.Number:
+		i, err := ethtypes.BigIntegerFromString(ctx, vt.String())
+		if err != nil {
+			return nil, i18n.WrapError(ctx, err, signermsgs.MsgInvalidIntegerABIInput, vt, v, desc)
+		}
+		return i, nil
 	case string:
-		// We use Go's default '0' base integer parsing, where `0x` means hex,
-		// no prefix means decimal etc.
-		i, ok := i.SetString(vt, 0)
-		if !ok {
-			f, _, err := big.ParseFloat(vt, 10, 256, big.ToNearestEven)
-			if err != nil {
-				return nil, i18n.NewError(ctx, signermsgs.MsgInvalidIntegerABIInput, vt, v, desc)
-			}
-			i, accuracy := f.Int(i)
-			if accuracy != big.Exact {
-				// If we weren't able to decode without losing precision, return an error
-				return nil, i18n.NewError(ctx, signermsgs.MsgInvalidIntegerABIInput, vt, v, desc)
-			}
-
-			return i, nil
+		i, err := ethtypes.BigIntegerFromString(ctx, vt)
+		if err != nil {
+			return nil, i18n.WrapError(ctx, err, signermsgs.MsgInvalidIntegerABIInput, vt, v, desc)
 		}
 		return i, nil
 	case *big.Float:
-		i, _ := vt.Int(i)
+		i, _ := vt.Int(nil)
 		return i, nil
 	case *big.Int:
 		return vt, nil
 	case float64:
 		// This is how JSON numbers come in (no distinction between integers/floats)
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetInt64(int64(vt)), nil
 	case float32:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetInt64(int64(vt)), nil
 	case int64:
-		i.SetInt64(vt)
-		return i, nil
+		return new(big.Int).SetInt64(vt), nil
 	case int32:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetInt64(int64(vt)), nil
 	case int16:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetInt64(int64(vt)), nil
 	case int8:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetInt64(int64(vt)), nil
 	case int:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetInt64(int64(vt)), nil
 	case uint64:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetUint64(vt), nil
 	case uint32:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetInt64(int64(vt)), nil
 	case uint16:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetInt64(int64(vt)), nil
 	case uint8:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetInt64(int64(vt)), nil
 	case uint:
-		i.SetInt64(int64(vt))
-		return i, nil
+		return new(big.Int).SetUint64(uint64(vt)), nil
 	default:
 		if str, ok := getStringIfConvertible(v); ok {
 			return getIntegerFromInterface(ctx, desc, str)
diff --git a/pkg/ethtypes/hexinteger.go b/pkg/ethtypes/hexinteger.go
index 5672d5c..c734c47 100644
--- a/pkg/ethtypes/hexinteger.go
+++ b/pkg/ethtypes/hexinteger.go
@@ -1,4 +1,4 @@
-// Copyright © 2023 Kaleido, Inc.
+// Copyright © 2024 Kaleido, Inc.
 //
 // SPDX-License-Identifier: Apache-2.0
 //
@@ -18,7 +18,6 @@ package ethtypes
 
 import (
 	"context"
-	"encoding/json"
 	"fmt"
 	"math/big"
 
@@ -37,25 +36,15 @@ func (h HexInteger) MarshalJSON() ([]byte, error) {
 }
 
 func (h *HexInteger) UnmarshalJSON(b []byte) error {
-	var i interface{}
-	_ = json.Unmarshal(b, &i)
-	switch i := i.(type) {
-	case float64:
-		*h = HexInteger(*big.NewInt(int64(i)))
-		return nil
-	case string:
-		bi, ok := new(big.Int).SetString(i, 0)
-		if !ok {
-			return fmt.Errorf("unable to parse integer: %s", i)
-		}
-		if bi.Sign() < 0 {
-			return fmt.Errorf("negative values are not supported: %s", i)
-		}
-		*h = HexInteger(*bi)
-		return nil
-	default:
-		return fmt.Errorf("unable to parse integer from type %T", i)
+	bi, err := UnmarshalBigInt(context.Background(), b)
+	if err != nil {
+		return err
+	}
+	if bi.Sign() < 0 {
+		return fmt.Errorf("negative values are not supported: %s", b)
 	}
+	*h = HexInteger(*bi)
+	return nil
 }
 
 func (h *HexInteger) BigInt() *big.Int {
diff --git a/pkg/ethtypes/hexinteger_test.go b/pkg/ethtypes/hexinteger_test.go
index 097f976..8a3d611 100644
--- a/pkg/ethtypes/hexinteger_test.go
+++ b/pkg/ethtypes/hexinteger_test.go
@@ -73,7 +73,10 @@ func TestHexIntegerMissingBytes(t *testing.T) {
 	}`
 
 	err := json.Unmarshal([]byte(testData), &testStruct)
-	assert.Regexp(t, "unable to parse integer", err)
+	assert.Regexp(t, "FF22088", err)
+
+	err = testStruct.I1.UnmarshalJSON([]byte(`{!badJSON`))
+	assert.Regexp(t, "invalid", err)
 }
 
 func TestHexIntegerBadType(t *testing.T) {
@@ -87,7 +90,7 @@ func TestHexIntegerBadType(t *testing.T) {
 	}`
 
 	err := json.Unmarshal([]byte(testData), &testStruct)
-	assert.Regexp(t, "unable to parse integer", err)
+	assert.Regexp(t, "FF22091", err)
 }
 
 func TestHexIntegerBadJSON(t *testing.T) {
diff --git a/pkg/ethtypes/hexuint64.go b/pkg/ethtypes/hexuint64.go
index 4c7f757..a9356fb 100644
--- a/pkg/ethtypes/hexuint64.go
+++ b/pkg/ethtypes/hexuint64.go
@@ -18,11 +18,11 @@ package ethtypes
 
 import (
 	"context"
-	"encoding/json"
 	"fmt"
 	"strconv"
 
 	"github.com/hyperledger/firefly-common/pkg/i18n"
+	"github.com/hyperledger/firefly-signer/internal/signermsgs"
 )
 
 // HexUint64 is a positive integer - serializes to JSON as an 0x hex string (no leading zeros), and parses flexibly depending on the prefix (so 0x for hex, or base 10 for plain string / float64)
@@ -40,22 +40,15 @@ func (h HexUint64) MarshalJSON() ([]byte, error) {
 }
 
 func (h *HexUint64) UnmarshalJSON(b []byte) error {
-	var i interface{}
-	_ = json.Unmarshal(b, &i)
-	switch i := i.(type) {
-	case float64:
-		*h = HexUint64(i)
-		return nil
-	case string:
-		i64, err := strconv.ParseUint(i, 0, 64)
-		if err != nil {
-			return fmt.Errorf("unable to parse integer: %s", i)
-		}
-		*h = HexUint64(i64)
-		return nil
-	default:
-		return fmt.Errorf("unable to parse integer from type %T", i)
+	bi, err := UnmarshalBigInt(context.Background(), b)
+	if err != nil {
+		return err
+	}
+	if !bi.IsUint64() {
+		return i18n.NewError(context.Background(), signermsgs.MsgInvalidUint64PrecisionLoss, b)
 	}
+	*h = HexUint64(bi.Uint64())
+	return nil
 }
 
 func (h HexUint64) Uint64() uint64 {
diff --git a/pkg/ethtypes/hexuint64_test.go b/pkg/ethtypes/hexuint64_test.go
index 3acdd78..7f246f8 100644
--- a/pkg/ethtypes/hexuint64_test.go
+++ b/pkg/ethtypes/hexuint64_test.go
@@ -73,7 +73,7 @@ func TestHexUint64MissingBytes(t *testing.T) {
 	}`
 
 	err := json.Unmarshal([]byte(testData), &testStruct)
-	assert.Regexp(t, "unable to parse integer", err)
+	assert.Regexp(t, "FF22088", err)
 }
 
 func TestHexUint64BadType(t *testing.T) {
@@ -87,7 +87,7 @@ func TestHexUint64BadType(t *testing.T) {
 	}`
 
 	err := json.Unmarshal([]byte(testData), &testStruct)
-	assert.Regexp(t, "unable to parse integer", err)
+	assert.Regexp(t, "FF22091", err)
 }
 
 func TestHexUint64BadJSON(t *testing.T) {
@@ -115,7 +115,21 @@ func TestHexUint64BadNegative(t *testing.T) {
 	}`
 
 	err := json.Unmarshal([]byte(testData), &testStruct)
-	assert.Regexp(t, "parse", err)
+	assert.Regexp(t, "FF22090", err)
+}
+
+func TestHexUint64BadTooLarge(t *testing.T) {
+
+	testStruct := struct {
+		I1 HexUint64 `json:"i1"`
+	}{}
+
+	testData := `{
+		"i1": "18446744073709551616"
+	}`
+
+	err := json.Unmarshal([]byte(testData), &testStruct)
+	assert.Regexp(t, "FF22090", err)
 }
 
 func TestHexUint64Constructor(t *testing.T) {
diff --git a/pkg/ethtypes/integer_parsing.go b/pkg/ethtypes/integer_parsing.go
new file mode 100644
index 0000000..f2f5427
--- /dev/null
+++ b/pkg/ethtypes/integer_parsing.go
@@ -0,0 +1,67 @@
+// Copyright © 2024 Kaleido, Inc.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ethtypes
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"math/big"
+
+	"github.com/hyperledger/firefly-common/pkg/i18n"
+	"github.com/hyperledger/firefly-common/pkg/log"
+	"github.com/hyperledger/firefly-signer/internal/signermsgs"
+)
+
+func BigIntegerFromString(ctx context.Context, s string) (*big.Int, error) {
+	// We use Go's default '0' base integer parsing, where `0x` means hex,
+	// no prefix means decimal etc.
+	i, ok := new(big.Int).SetString(s, 0)
+	if !ok {
+		f, _, err := big.ParseFloat(s, 10, 256, big.ToNearestEven)
+		if err != nil {
+			log.L(ctx).Errorf("Error parsing numeric string '%s': %s", s, err)
+			return nil, i18n.NewError(ctx, signermsgs.MsgInvalidNumberString, s)
+		}
+		i, accuracy := f.Int(i)
+		if accuracy != big.Exact {
+			// If we weren't able to decode without losing precision, return an error
+			return nil, i18n.NewError(ctx, signermsgs.MsgInvalidIntPrecisionLoss, s)
+		}
+
+		return i, nil
+	}
+	return i, nil
+}
+
+func UnmarshalBigInt(ctx context.Context, b []byte) (*big.Int, error) {
+	var i interface{}
+	d := json.NewDecoder(bytes.NewReader(b))
+	d.UseNumber()
+	err := d.Decode(&i)
+	if err != nil {
+		return nil, err
+	}
+	switch i := i.(type) {
+	case json.Number:
+		return BigIntegerFromString(context.Background(), i.String())
+	case string:
+		return BigIntegerFromString(context.Background(), i)
+	default:
+		return nil, i18n.NewError(ctx, signermsgs.MsgInvalidJSONTypeForBigInt, i)
+	}
+}
diff --git a/pkg/ethtypes/integer_parsing_test.go b/pkg/ethtypes/integer_parsing_test.go
new file mode 100644
index 0000000..b5e4c06
--- /dev/null
+++ b/pkg/ethtypes/integer_parsing_test.go
@@ -0,0 +1,46 @@
+// Copyright © 2024 Kaleido, Inc.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ethtypes
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIntegerParsing(t *testing.T) {
+	ctx := context.Background()
+
+	i, err := BigIntegerFromString(ctx, "1.0000000000000000000000001e+25")
+	assert.NoError(t, err)
+	assert.Equal(t, "10000000000000000000000001", i.String())
+
+	i, err = BigIntegerFromString(ctx, "10000000000000000000000000000001")
+	assert.NoError(t, err)
+	assert.Equal(t, "10000000000000000000000000000001", i.String())
+
+	i, err = BigIntegerFromString(ctx, "20000000000000000000000000000002")
+	assert.NoError(t, err)
+	assert.Equal(t, "20000000000000000000000000000002", i.String())
+
+	_, err = BigIntegerFromString(ctx, "0xGG")
+	assert.Regexp(t, "FF22088", err)
+
+	_, err = BigIntegerFromString(ctx, "3.0000000000000000000000000000003")
+	assert.Regexp(t, "FF22089", err)
+}