Phenylhydrazones of carbonyl compounds C6H5-CO-R (R = H, Me, Et, Ph) have been subjected to vapour-phase pyrolysis at temperatures ranging from 650 to 1100�K. Analysis of the products indicates that a four-centre reaction is involved in which the group R migrates to the α- nitrogen of the phenylhydrazone moiety, with a concomitant production of benzonitrile. The mechanisms of this and competing reactions are discussed.