The present invention relates to a process for the preparation of iodinated X-ray contrast agents and in particular to key intermediates thereof. More particularly, the invention relates to a work-up process for preparation of a compound mixture comprising 1-formylamino-3,5-bis(2,3-bis(formyloxy)propan-1-ylcarbamoyl)-2,4,6-trioodobenzene, a key intermediate in the process for preparing Ioforminol. Further, the invention relates to a process for preparing Ioforminol, a contrast agent useful in X-ray imaging.