diff --git a/tests/test_nested_arrays.py b/tests/test_nested_arrays.py
new file mode 100644
index 0000000..788fc9c
--- /dev/null
+++ b/tests/test_nested_arrays.py
@@ -0,0 +1,183 @@
+"""Tests for nested arrays and objects within tabular arrays (Issue #6)."""
+import pytest
+from toon import encode, decode
+
+
+def test_array_of_objects_with_nested_arrays():
+    """Test encoding/decoding arrays of objects that contain nested arrays."""
+    original = {
+        "categorization": [
+            {
+                "id": "01.04.04.01.",
+                "label": "Aspetti generali",
+                "hierarchy": [
+                    "Prodotti",
+                    "Organizzazione altro e Sito Internet",
+                    "Aspetti generali",
+                    "Aspetti generali"
+                ],
+                "score": 900,
+                "winner": True,
+                "namespace": "$namespace",
+                "frequency": 0,
+                "offset": [
+                    {"start": 511, "end": 520},
+                    {"start": 524, "end": 527},
+                    {"start": 528, "end": 543}
+                ]
+            }
+        ]
+    }
+    
+    # Encode
+    toon = encode(original)
+    print("Encoded TOON:")
+    print(toon)
+    
+    # Decode
+    result = decode(toon)
+    print("\nDecoded result:")
+    print(result)
+    
+    # Verify all fields are preserved
+    assert result == original, "Decoded data should match original"
+    assert "hierarchy" in result["categorization"][0], "hierarchy field should be preserved"
+    assert "offset" in result["categorization"][0], "offset field should be preserved"
+    assert len(result["categorization"][0]["hierarchy"]) == 4, "hierarchy array should have 4 items"
+    assert len(result["categorization"][0]["offset"]) == 3, "offset array should have 3 items"
+
+
+def test_array_of_objects_with_nested_objects():
+    """Test encoding/decoding arrays where objects contain nested objects."""
+    original = {
+        "users": [
+            {
+                "id": 1,
+                "name": "Alice",
+                "address": {
+                    "street": "123 Main St",
+                    "city": "NYC"
+                }
+            },
+            {
+                "id": 2,
+                "name": "Bob",
+                "address": {
+                    "street": "456 Oak Ave",
+                    "city": "LA"
+                }
+            }
+        ]
+    }
+    
+    # Encode
+    toon = encode(original)
+    print("Encoded TOON:")
+    print(toon)
+    
+    # Decode
+    result = decode(toon)
+    
+    # Verify all fields are preserved
+    assert result == original
+    assert "address" in result["users"][0]
+    assert result["users"][0]["address"]["city"] == "NYC"
+
+
+def test_array_of_objects_mixed_primitive_and_nested():
+    """Test arrays with both primitive and nested fields."""
+    original = {
+        "items": [
+            {
+                "id": 1,
+                "name": "Item A",
+                "tags": ["tag1", "tag2"],
+                "price": 10.5
+            },
+            {
+                "id": 2,
+                "name": "Item B",
+                "tags": ["tag3"],
+                "price": 20.0
+            }
+        ]
+    }
+    
+    # Encode
+    toon = encode(original)
+    
+    # Decode
+    result = decode(toon)
+    
+    # Verify all fields are preserved
+    assert result == original
+    assert "tags" in result["items"][0]
+    assert len(result["items"][0]["tags"]) == 2
+    assert len(result["items"][1]["tags"]) == 1
+
+
+def test_roundtrip_complex_nested_structure():
+    """Test full roundtrip of complex nested structure."""
+    original = {
+        "data": [
+            {
+                "id": "A1",
+                "value": 100,
+                "metadata": {
+                    "created": "2024-01-01",
+                    "tags": ["important", "urgent"]
+                },
+                "scores": [95, 87, 92]
+            },
+            {
+                "id": "A2", 
+                "value": 200,
+                "metadata": {
+                    "created": "2024-01-02",
+                    "tags": ["normal"]
+                },
+                "scores": [88, 90]
+            }
+        ]
+    }
+    
+    # First roundtrip
+    toon1 = encode(original)
+    result1 = decode(toon1)
+    assert result1 == original
+    
+    # Second roundtrip
+    toon2 = encode(result1)
+    result2 = decode(toon2)
+    assert result2 == original
+    assert toon1 == toon2
+
+
+def test_array_of_objects_some_with_nested_some_without():
+    """Test arrays where only some objects have nested fields."""
+    original = {
+        "records": [
+            {
+                "id": 1,
+                "name": "Record A",
+                "extra": {"note": "Has nested"}
+            },
+            {
+                "id": 2,
+                "name": "Record B"
+                # No 'extra' field
+            }
+        ]
+    }
+    
+    # Encode
+    toon = encode(original)
+    
+    # Decode
+    result = decode(toon)
+    
+    # Verify structure is preserved
+    assert "extra" in result["records"][0]
+    assert "extra" not in result["records"][1]
+    assert result["records"][0]["extra"]["note"] == "Has nested"
+
diff --git a/toon/utils.py b/toon/utils.py
index 9f85722..29e7ce6 100644
--- a/toon/utils.py
+++ b/toon/utils.py
@@ -158,33 +158,47 @@ def is_array_of_objects(value: Any) -> bool:
 
 def is_uniform_array_of_objects(value: list) -> Optional[list]:
     """
-    Check if an array contains objects with identical primitive fields.
+    Check if an array contains objects with identical primitive-only fields.
+    
+    This function determines if an array of objects can use the compact tabular format.
+    Tabular format is only used when ALL fields in ALL objects are primitive types.
+    If any object contains non-primitive fields (arrays, nested objects), the function
+    returns None, and the encoder will use list array format instead to preserve all data.
     
     Args:
         value: Array to check
         
     Returns:
-        List of field names if uniform, None otherwise
+        List of field names if uniform and all primitive, None otherwise
     """
     if not value or not all(isinstance(item, dict) for item in value):
         return None
     
-    # Get fields from first object
+    # Get all fields from first object and check if they're primitive
     first_obj = value[0]
     fields = []
     
     for key, val in first_obj.items():
-        if is_primitive(val):
-            fields.append(key)
+        if not is_primitive(val):
+            # Object contains non-primitive field (array or nested object)
+            # Cannot use tabular format - must use list format to preserve all data
+            return None
+        fields.append(key)
     
     if not fields:
         return None
     
-    # Check all objects have the same primitive fields
+    # Check all objects have the exact same fields, all primitive
     for obj in value[1:]:
-        obj_fields = [k for k, v in obj.items() if is_primitive(v)]
-        if set(obj_fields) != set(fields):
+        # Check that this object has exactly the same fields
+        if set(obj.keys()) != set(fields):
             return None
+        
+        # Check that all values in this object are primitive
+        for key, val in obj.items():
+            if not is_primitive(val):
+                # Found non-primitive field - cannot use tabular format
+                return None
     
     return fields