The invention relates to industrial preparation of contrast agents, and further to an improved process for the purification of contrast agents. In particular, it relates to a process for reducing the salt content of compositions comprising an MR contrast agent or an X-ray contrast agent, such as a non-ionic iodinated monomeric compound or a non-ionic iodinated dimeric compound.本発明は、造影剤の工業的調製、さらに造影剤の改良精製方法に関する。特に、本発明は、非イオン性ヨウ素化モノマー化合物又は非イオン性ヨウ素化ダイマー化合物などのMR造影剤又はX線造影剤を含む組成物の塩分含有量を低減する方法に関する。