When titrated potentiometrically in acetonitrile 1 mol of phenylhydrazine reacts with 1 mol of perchloric acid, whereas 1 mol of 1,2-diphenylhydrazine reacts with 2 mol of perchloric acid. This is an indication that the benzidine rearrangement reaction occurs under these conditions. In order to support this view, the salts formed during the titration were filtered off and neturalised with diethylamine. The neutralised material was recrystallised from pure ethanol and was shown to be benzidine by infrared and UV-visible spectrophotometry. Benzidine is formed when 1,2-diphenylhydrazine undergoes the benzidine rearrangement reaction. Chromatographic methods were also used to explain, at least in part, the mechanism of the reaction. From the results obtained it was concluded that the benzidine rearrangement reaction was occurring and that it proceeded through a monoprotonated 1,2-diphenylhydrazine species rather than a diprotonated species as proposed by other workers. This is the first benzidine rearrangement reaction to be observed in a non-aqueous medium such as acetonitrile.