The kinetics of thermal isomerization of 3,3-dihalo-1,2-diphenylaziridines containing fluorine, chlorine, or bromine atoms in the ring were studied, and the activation parameters of the reaction were determined. A concerted mechanism for the formation of imidoyl halides from haloaziridines that includes conrotatory opening of the aziridine ring with halogen migration of the 1,2 type is proposed on the basis of the data obtained.