Only if your ex made after-tax payments to the plan would any portion of the distribution be nontaxable. With the pension split under a QDRO, your ex's after-tax basis, if any, generally would be split in proportion to the overall split and your share of that after-tax basis would be your Plan Cost. Since the payer doesn't seem to know if there is any after-tax basis in the plan, you would need to obtain that information from your ex.
