diff --git a/examples/avb2aaf.py b/examples/avb2aaf.py
index cfb2b79..a66a5ce 100644
--- a/examples/avb2aaf.py
+++ b/examples/avb2aaf.py
@@ -284,7 +284,7 @@ def check_source_clip(f, avb_source_clip):
     mob_id = MobID(bytes_le=avb_source_clip.mob_id.bytes_le)
     if zero_mob_id(mob_id):
         return 0
-    avb_mob = avb_source_clip.root.content.mob_dict[avb_source_clip.mob_id]
+    avb_mob = avb_source_clip.root.content.find_by_mob_id(avb_source_clip.mob_id)
     if mob_id in f.content.mobs:
         mob = f.content.mobs[mob_id]
     else:
@@ -797,7 +797,6 @@ def avb2aaf_main(path):
     with avb.open(path) as avb_file:
         with aaf2.open(path + ".aaf", 'w') as aaf_file:
             register_definitions(aaf_file)
-            avb_file.content.build_mob_dict()
             avb2aaf(aaf_file, avb_file)
 
     # with aaf2.open(path + ".aaf", 'r') as aaf_file:
diff --git a/src/avb/bin.py b/src/avb/bin.py
index f4876cb..5de8841 100644
--- a/src/avb/bin.py
+++ b/src/avb/bin.py
@@ -216,7 +216,7 @@ class Bin(core.AVBObject):
         AVBPropertyDef('attributes',       'BinAttr',        'reference'),
         AVBPropertyDef('was_iconic',       'WasIconic',      'bool',      False),
     ]
-    __slots__ = ('mob_dict')
+    __slots__ = ('_mob_dict')
 
     def __init__(self):
         super(Bin, self).__init__(self)
@@ -232,7 +232,7 @@ def __init__(self):
             self.sifted_settings.append(s)
 
         self.attributes = self.root.create.Attributes()
-        self.mob_dict ={}
+        self._mob_dict ={}
 
     def read(self, f):
         super(Bin, self).read(f)
@@ -256,7 +256,7 @@ def read(self, f):
             object_count = ctx.read_u32(f)
 
         self.items = []
-        self.mob_dict ={}
+        self._mob_dict ={}
 
         for i in range(object_count):
             bin_obj = BinItem.__new__(BinItem, root=self.root)
@@ -369,16 +369,25 @@ def write(self, f):
         if self.class_id[:] == b'ABIN':
             ctx.write_u8(f, 0x03)
 
-    def build_mob_dict(self):
-        self.mob_dict = {}
-        for mob in self.mobs:
-            self.mob_dict[mob.mob_id] = mob
+    def find_by_mob_id(self, mob_id):
+
+        # build mob_id dictionary
+        if not self._mob_dict:
+            for mob in self.mobs:
+                self._mob_dict[mob.mob_id] = mob
+
+        return self._mob_dict.get(mob_id, None)
 
     def add_mob(self, mob):
         bin_item = self.root.create.BinItem()
         bin_item.mob = mob
         self.items.append(bin_item)
 
+        if self._mob_dict:
+            self._mob_dict[mob.mob_id] = mob
+
+        return bin_item
+
     @property
     def mobs(self):
         for item in self.items:
diff --git a/src/avb/components.py b/src/avb/components.py
index 7ede820..a393c1b 100644
--- a/src/avb/components.py
+++ b/src/avb/components.py
@@ -313,13 +313,10 @@ def write(self, f):
 
     @property
     def mob(self):
-        if not self.root.content.mob_dict:
-            self.root.content.build_mob_dict()
-
         if hasattr(self, 'mob_id'):
             mob_id = self.mob_id
             if mob_id:
-                return self.root.content.mob_dict.get(self.mob_id, None)
+                return self.root.content.find_by_mob_id(self.mob_id)
 
     @property
     def track(self):
diff --git a/src/avb/trackgroups.py b/src/avb/trackgroups.py
index c951afb..8a8b8d9 100644
--- a/src/avb/trackgroups.py
+++ b/src/avb/trackgroups.py
@@ -233,9 +233,6 @@ def dependant_mobs(self):
         visited = set()
         stack = [self]
 
-        if not self.root.content.mob_dict:
-            self.root.content.build_mob_dict()
-
         while stack:
             mob = stack[-1]
             children_processed = True
diff --git a/tests/test_copy.py b/tests/test_copy.py
index 455f3cf..314f138 100644
--- a/tests/test_copy.py
+++ b/tests/test_copy.py
@@ -31,9 +31,9 @@ def test_copy(self):
 
         with avb.open(test_file_01) as a:
             with avb.open(result_file) as b:
-                b.content.build_mob_dict()
+
                 for mob_a in a.content.mobs:
-                    mob_b = b.content.mob_dict.get(mob_a.mob_id, None)
+                    mob_b = b.content.find_by_mob_id(mob_a.mob_id)
                     assert mob_b
                     compare(mob_a, mob_b)
 
@@ -59,11 +59,9 @@ def test_copy_mastermob_depends(self):
 
         with avb.open(test_file_01) as a:
             with avb.open(result_file) as b:
-                b.content.build_mob_dict()
-                a.content.build_mob_dict()
                 for mob_id in mob_ids:
-                    mob_a = a.content.mob_dict.get(mob_id, None)
-                    mob_b = b.content.mob_dict.get(mob_id, None)
+                    mob_a = a.content.find_by_mob_id(mob_id)
+                    mob_b = b.content.find_by_mob_id(mob_id)
                     compare(mob_a, mob_b)
 
     def test_copy_compositionmobs(self):
@@ -89,11 +87,9 @@ def test_copy_compositionmobs(self):
 
         with avb.open(test_file_01) as a:
             with avb.open(result_file) as b:
-                b.content.build_mob_dict()
-                a.content.build_mob_dict()
                 for mob_id in mob_ids:
-                    mob_a = a.content.mob_dict.get(mob_id, None)
-                    mob_b = b.content.mob_dict.get(mob_id, None)
+                    mob_a = a.content.find_by_mob_id(mob_id)
+                    mob_b = b.content.find_by_mob_id(mob_id)
                     compare(mob_a, mob_b)
 
 if __name__ == "__main__":