diff --git a/src/documents/serialisers.py b/src/documents/serialisers.py
index 87c6657da..3a7f505db 100644
--- a/src/documents/serialisers.py
+++ b/src/documents/serialisers.py
@@ -646,10 +646,26 @@ class CustomFieldInstanceSerializer(serializers.ModelSerializer):
             custom_field.data_type,
         )
 
+        api_version = int(
+            self.context.get("request").version
+            if self.context.get("request")
+            else settings.REST_FRAMEWORK["ALLOWED_VERSIONS"][-1],
+        )
+
         if custom_field.data_type == CustomField.FieldDataType.DOCUMENTLINK:
             # prior to update so we can look for any docs that are going to be removed
             self.reflect_doclinks(document, custom_field, validated_data["value"])
 
+        if (
+            custom_field.data_type == CustomField.FieldDataType.SELECT
+            and api_version < 7
+        ):
+            # Convert the index of the option in the field.extra_data["select_options"] list
+            # to the actual value
+            validated_data["value"] = custom_field.extra_data["select_options"][
+                validated_data["value"]
+            ]["id"]
+
         # Actually update or create the instance, providing the value
         # to fill in the correct attribute based on the type
         instance, _ = CustomFieldInstance.objects.update_or_create(
@@ -660,6 +676,21 @@ class CustomFieldInstanceSerializer(serializers.ModelSerializer):
         return instance
 
     def get_value(self, obj: CustomFieldInstance):
+        api_version = int(
+            self.context.get("request").version
+            if self.context.get("request")
+            else settings.REST_FRAMEWORK["ALLOWED_VERSIONS"][-1],
+        )
+        if api_version < 7 and obj.field.data_type == CustomField.FieldDataType.SELECT:
+            # return the index of the option in the field.extra_data["select_options"] list
+            return next(
+                (
+                    idx
+                    for idx, option in enumerate(obj.field.extra_data["select_options"])
+                    if option["id"] == obj.value
+                ),
+                None,
+            )
         return obj.value
 
     def validate(self, data):
@@ -958,6 +989,12 @@ class DocumentSerializer(
         ):
             kwargs["full_perms"] = True
 
+        self.api_version = int(
+            context.get("request").version
+            if context.get("request")
+            else settings.REST_FRAMEWORK["ALLOWED_VERSIONS"][-1],
+        )
+
         super().__init__(*args, **kwargs)
 
     class Meta:
diff --git a/src/documents/tests/test_api_custom_fields.py b/src/documents/tests/test_api_custom_fields.py
index cd15de006..e4a261ff7 100644
--- a/src/documents/tests/test_api_custom_fields.py
+++ b/src/documents/tests/test_api_custom_fields.py
@@ -328,6 +328,60 @@ class TestCustomFieldsAPI(DirectoriesMixin, APITestCase):
             ],
         )
 
+    def test_custom_field_select_value_old_version(self):
+        """
+        GIVEN:
+            - Existing document with custom field select
+        WHEN:
+            - API post request is made to add the field for document with api version header < 7
+            - API get request is made for document with api version header < 7
+        THEN:
+            - The select value is returned in the old format, the index of the option
+        """
+        custom_field_select = CustomField.objects.create(
+            name="Select Field",
+            data_type=CustomField.FieldDataType.SELECT,
+            extra_data={
+                "select_options": [
+                    {"label": "Option 1", "id": "abc-123"},
+                    {"label": "Option 2", "id": "def-456"},
+                ],
+            },
+        )
+
+        doc = Document.objects.create(
+            title="WOW",
+            content="the content",
+            checksum="123",
+            mime_type="application/pdf",
+        )
+        CustomFieldInstance.objects.create(
+            document=doc,
+            field=custom_field_select,
+            value_select="abc-123",
+        )
+
+        resp = self.client.patch(
+            f"/api/documents/{doc.id}/",
+            headers={"Accept": "application/json; version=6"},
+            data={
+                "custom_fields": [
+                    {"field": custom_field_select.id, "value": 0},
+                ],
+            },
+        )
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+        self.assertEqual(doc.custom_fields.first().value, "abc-123")
+
+        resp = self.client.get(
+            f"/api/documents/{doc.id}/",
+            headers={"Accept": "application/json; version=6"},
+        )
+        self.assertEqual(resp.status_code, status.HTTP_200_OK)
+
+        data = resp.json()
+        self.assertEqual(data["custom_fields"][0]["value"], 0)
+
     def test_create_custom_field_monetary_validation(self):
         """
         GIVEN: