Unsymmetric di(heteroaryl)ureas such as HetAr1-NHCONH-HetAr2 are efficiently synthesized from two symmetricureas, HetAr1-NHCONH-HetAr1 and HetAr2-NHCONH-HetAr2, by rhodium-catalyzed exchange reactions. The equilibrium in some of the reactions can be shifted to the formation of unsymmetricureas by the aggregation of the dimers formed by inter- and intramolecular hydrogen bonding.