The present invention relates to a class of compounds and to diagnostic compositions containing such compounds where the compounds are iodine containing compounds of the formula R-Y-X-Z-R where each R denotes a triiodinated phenyl residue further substituted by hydrophilic moieties, Y and Z are urea and urethane groups and X is a alkylene group which may be further substituted. The invention also relates to the use of such diagnostic compositions as contrast agents in diagnostic imaging and in particular in X-ray imaging, and to contrast media containing such compounds.