The mass spectra of a number of substituted aroylhydrazones, ArCO·NH·NC(R)Ar′ where R = H or alkyl, are reported. Fragmentation of the hydrazones with R = H is dominated by paths leading to the ions ArCO^+′ArCO·NH₂^+˙, and Ar′CN^+˙, and evidence is presented that the hydrogen rearrangement to form ArCO·NH₂^+˙ proceeds via a four-membered transition state. Formation of the ion ArCN^+˙ was shown to be due to thermal decomposition of the hydrazones in the ion chamber. Hydrazones with R = alkyl and two diaroylhydrazones, ArCO(Ar′CO)N·NCHPh, were found to undergo skeletal rearrangement in addition to simple bond cleavage.