Bis(benzonitrile)palladium dichloride is an effective catalyst for the cycloaddition reaction of aziridines with carbodiimides to form imidazolidenimines in 40-94% yields. The process is a regiospecific one, involving cleavage of the more substituted ring carbon-nitrogen bond. An X-ray structure determination of one of the imidazolidinimines, together with spectral and analytical data for the products of all the cycloaddition reactions, provided the basis for the structure assignment.