A study of the ring-opening reaction of 2,2-gem-diphenylaziridine by treating with acetic acid has been undertaken. The structure of the ring-opened product was confirmed as 1,1-diphenyl-2-aminoethyl acetate. It is most likely that the reaction proceeds through the cleavage of a bond between nitrogen and tertiary carbon atoms in the aziridine ring, followed by the formation of a carbonium ion intermediate.