Assuming that the mortgage is a joint one, there isn't an 'his half of the mortgage', you are both jointly and severally liable for the full amount. The lender doesn't care who pays it, as long as one of you does. If you want to keep your credit record clean, you will have to pay it. Otherwise advise your ex that if neither of you pay, it will probably end up getting repossessed and will definitely trash both your credit files for a very long time.