Hello, I found an extra couple of brain cells and got it working. Posting my efforts for any others who might end up here. This is C# code in .Net 3.5. //Get the profile record ArbApiSoap.GetCustomerProfileResponseType readProfile = Service.GetCustomerProfile(MerchantAuthentication, customer.AuthorizeDotNetCustomerProfileID); //Create a "write" profile record ArbApiSoap.CustomerProfileExType writeProfile = new CustomerProfileExType(); //Make the "write" look like the "read" writeProfile.customerProfileId = readProfile.profile.customerProfileId; writeProfile.description = readProfile.profile.description; writeProfile.email = customer.Email; writeProfile.merchantCustomerId = readProfile.profile.merchantCustomerId; //Update the profile ArbApiSoap.UpdateCustomerProfileResponseType resp = Service.UpdateCustomerProfile(MerchantAuthentication, writeProfile); //Act on "resp"...