Your spouse is not liable for your separate tax debts - regardless of whether the debts are prenuptial or postnuptial. If her prenuptial assets have remained separate property (deed, title, account is in her name and the assets haven't been mixed together with marital assets), then the assets remain her separate property and they can't be applied to your separate tax debt. If the property lost it separate status, then the postnuptial agreement would not be effective, because it would be viewed under the law as a gift from your to your spouse and gifted property can be clawed back by the IRS.
Answered on May 22nd, 2015 at 12:10 AM