A process for the preparation of iodixanol by dimerization of 5-acetamido-N,N-bis(2,3-dihydroxypropyl)-2,4,6-triiodo-isophthalamide ("Compound A") in which, after the dimerization step, unreacted Compound A is precipitated from the reaction mixture and recovered for re-use. The process substantially increases the net yield of iodixanol and simplifies its purification.