The reaction of methylhydrazones 1 with nitrile oxide has been examined. The initial product is the Z-adduct 2, which, depending on the reaction procedure and the substituents, can undergo either isomerization to the thermodynamically stable E-adduct 3 or tautomerization to the oxatriazine 4 or irreversible cyclization to the triazole 5. A general mechanism consistent with these results is proposed. Structure 4 was confirmed by X-ray studies.