Hello RonaldA9,   Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.   Using the following methods to update contacts when setting a custom field value to null will result as:   PUT https://api.cc.email/v3/contacts/{contact_id} Updating a single existing contact using the Contacts endpoint with a custom field value set to null will remove that custom value from that contact. You can also omit the custom field from the request body for the same result, as a PUT call on the Contacts endpoint overwrites all existing data for the contact with whatever you put in the request body.   POST https://api.cc.email/v3/contacts/sign_up_form Creating or updating a single contact using the Sign-up form endpoint with a custom field value set to null will remove that custom value from that contact. Omitting the custom field will make no change, as this method appends data.   POST https://api.cc.email/v3/activities/contacts_json_import  Creating or updating a single contact using the Bulk Activities endpoints with a custom field value set to null will NOT make any change to the  custom value from that contact, it will be ignored. This is due to the way that this endpoint appends data.   Please have a look and let us know if you have any other questions! ... View more