diff --git a/OpenMcdf.Ole.Tests/OlePropertiesExtensionsTests.cs b/OpenMcdf.Ole.Tests/OlePropertiesExtensionsTests.cs
index e92126d5..b2311edf 100644
--- a/OpenMcdf.Ole.Tests/OlePropertiesExtensionsTests.cs
+++ b/OpenMcdf.Ole.Tests/OlePropertiesExtensionsTests.cs
@@ -327,6 +327,88 @@ public void AddDocumentSummaryInformationCustomInfo()
         }
     }
 
+    /// As Test_DOCUMENT_SUMMARY_INFO_ADD_CUSTOM, but adding user defined properties with the AddUserDefinedProperty function
+    [TestMethod]
+    public void TestAddUserDefinedProperty()
+    {
+        using MemoryStream modifiedStream = new();
+        using (FileStream stream = File.OpenRead("english.presets.doc"))
+            stream.CopyTo(modifiedStream);
+
+        // Test value for a VT_FILETIME property
+        DateTime testNow = DateTime.UtcNow;
+
+        // english.presets.doc has a user defined property section, but no properties other than the codepage
+        using (var cf = RootStorage.Open(modifiedStream, StorageModeFlags.LeaveOpen))
+        {
+            CfbStream dsiStream = cf.OpenStream("\u0005DocumentSummaryInformation");
+            OlePropertiesContainer co = new(dsiStream);
+            OlePropertiesContainer userProperties = co.UserDefinedProperties!;
+            userProperties.AddUserDefinedProperty(VTPropertyType.VT_LPSTR, "StringProperty").Value = "Hello";
+            userProperties.AddUserDefinedProperty(VTPropertyType.VT_BOOL, "BooleanProperty").Value = true;
+            userProperties.AddUserDefinedProperty(VTPropertyType.VT_I4, "IntegerProperty").Value = 3456;
+            userProperties.AddUserDefinedProperty(VTPropertyType.VT_FILETIME, "DateProperty").Value = testNow;
+            userProperties.AddUserDefinedProperty(VTPropertyType.VT_R8, "DoubleProperty").Value = 1.234567d;
+
+            co.Save(dsiStream);
+        }
+
+        ValidateAddedUserDefinedProperties(modifiedStream, testNow);
+    }
+
+    // Validate that the user defined properties added by Test_DOCUMENT_SUMMARY_INFO_ADD_CUSTOM / Test_Add_User_Defined_Property are as expected
+    private static void ValidateAddedUserDefinedProperties(MemoryStream stream, DateTime testFileTimeValue)
+    {
+        using var cf = RootStorage.Open(stream);
+        using CfbStream cfbStream = cf.OpenStream("\u0005DocumentSummaryInformation");
+        OlePropertiesContainer co = new(cfbStream);
+        IList<OleProperty> propArray = co.UserDefinedProperties!.Properties;
+        Assert.AreEqual(6, propArray.Count);
+
+        // CodePage prop
+        Assert.AreEqual(1u, propArray[0].PropertyIdentifier);
+        Assert.AreEqual("0x00000001", propArray[0].PropertyName);
+        Assert.AreEqual((short)-535, propArray[0].Value);
+
+        // User properties
+        Assert.AreEqual("StringProperty", propArray[1].PropertyName);
+        Assert.AreEqual("Hello", propArray[1].Value);
+        Assert.AreEqual(VTPropertyType.VT_LPSTR, propArray[1].VTType);
+        Assert.AreEqual("BooleanProperty", propArray[2].PropertyName);
+        Assert.AreEqual(true, propArray[2].Value);
+        Assert.AreEqual(VTPropertyType.VT_BOOL, propArray[2].VTType);
+        Assert.AreEqual("IntegerProperty", propArray[3].PropertyName);
+        Assert.AreEqual(3456, propArray[3].Value);
+        Assert.AreEqual(VTPropertyType.VT_I4, propArray[3].VTType);
+        Assert.AreEqual("DateProperty", propArray[4].PropertyName);
+        Assert.AreEqual(testFileTimeValue, propArray[4].Value);
+        Assert.AreEqual(VTPropertyType.VT_FILETIME, propArray[4].VTType);
+        Assert.AreEqual("DoubleProperty", propArray[5].PropertyName);
+        Assert.AreEqual(1.234567d, propArray[5].Value);
+        Assert.AreEqual(VTPropertyType.VT_R8, propArray[5].VTType);
+    }
+
+    /// The names of user defined properties must be unique - adding a duplicate should throw.
+    [TestMethod]
+    public void TestAddUserDefinedPropertyShouldPreventDuplicates()
+    {
+        using MemoryStream modifiedStream = new();
+        using (FileStream stream = File.OpenRead("english.presets.doc"))
+            stream.CopyTo(modifiedStream);
+
+        using var cf = RootStorage.Open(modifiedStream);
+        CfbStream dsiStream = cf.OpenStream("\u0005DocumentSummaryInformation");
+        OlePropertiesContainer co = new(dsiStream);
+        OlePropertiesContainer userProperties = co.UserDefinedProperties!;
+
+        userProperties.AddUserDefinedProperty(VTPropertyType.VT_LPSTR, "StringProperty");
+
+        ArgumentException exception = Assert.ThrowsException<ArgumentException>(
+            () => userProperties.AddUserDefinedProperty(VTPropertyType.VT_LPSTR, "stringproperty"));
+
+        Assert.AreEqual("name", exception.ParamName);
+    }
+
     // Try to read a document which contains Vector/String properties
     // refs https://github.com/ironfede/openmcdf/issues/98
     [TestMethod]
diff --git a/OpenMcdf.Ole/OlePropertiesContainer.cs b/OpenMcdf.Ole/OlePropertiesContainer.cs
index 8c8d564c..f6f66be7 100644
--- a/OpenMcdf.Ole/OlePropertiesContainer.cs
+++ b/OpenMcdf.Ole/OlePropertiesContainer.cs
@@ -127,6 +127,49 @@ public OleProperty CreateProperty(VTPropertyType vtPropertyType, uint propertyId
 
     public void Add(OleProperty property) => properties.Add(property);
 
+    /// <summary>
+    /// Create a new UserDefinedProperty.
+    /// </summary>
+    /// <param name="vtPropertyType">The type of property to create.</param>
+    /// <param name="name">The name of the new property.</param>
+    /// <returns>The new property.</returns>
+    /// <exception cref="InvalidOperationException">If UserDefinedProperties aren't allowed for this container.</exception>
+    /// <exception cref="ArgumentException">If a property with the name <paramref name="name"/> already exists."/></exception>
+    public OleProperty AddUserDefinedProperty(VTPropertyType vtPropertyType, string name)
+    {
+        if (this.ContainerType != ContainerType.UserDefinedProperties)
+            throw new InvalidOperationException();
+
+        // As per https://learn.microsoft.com/en-us/openspecs/windows_protocols/MS-OLEPS/4177a4bc-5547-49fe-a4d9-4767350fd9cf
+        // the property names have to be unique, and are case insensitive.
+        if (PropertyNames.Any(property => property.Value.Equals(name, StringComparison.InvariantCultureIgnoreCase)))
+        {
+            throw new ArgumentException($"User defined property names must be unique and {name} already exists.", nameof(name));
+        }
+
+        // Work out a property identifier - must be > 1 and unique as per
+        // https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-oleps/333959a3-a999-4eca-8627-48a224e63e77
+        uint identifier = 2;
+
+        if (PropertyNames.Count > 0)
+        {
+            uint highestIdentifier = PropertyNames.Keys.Max();
+            identifier = Math.Max(highestIdentifier, 2) + 1;
+        }
+
+        PropertyNames[identifier] = name;
+
+        var op = new OleProperty(this)
+        {
+            VTType = vtPropertyType,
+            PropertyIdentifier = identifier
+        };
+
+        properties.Add(op);
+
+        return op;
+    }
+
     public void RemoveProperty(uint propertyIdentifier)
     {
         OleProperty? toRemove = properties.FirstOrDefault(o => o.PropertyIdentifier == propertyIdentifier);