The invention provides a method of purifying 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophathalamide, comprising adding a portion of acid to a stream comprising crude 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophathalamide in a solvent and continuously crystallising the 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophathalamide from the crude 5-acetamido-N,N′-bis(2,3-dihydroxypropyl)-2,4,6-triiodoisophathalamide, with removal of at least a fraction of the solvent, wherein the crystallisation is carried out in a continuous reactor, wherein the continuous reactor is a plug flow reactor. At least three further portions of acid are added to the continuous reactor during the crystallisation.