Bis(benzonitrile)palladium dichloride is an effective catalyst for the cyclization reaction of aziridines and sulfur diimides, in toluene, affording imidazolidinathiones in 52-70% yield. Reaction of an aziridine, labeled with C-13 at one of the ring carbons, with a sulfur diimide resulted in incorporation of the label at the 2- and 5-positions of the imidazolidinethione. Thiazolidinimine formation results from the palladium(II)-catalyzed reaction of an aziridine with phenyl isothiocyanate.