Depending on the substitution on the hydrazine moiety, thermolysis of 3-phenyl-1,2,4-oxadiazol-5-ylhydrazines (1)–(5) gives variable amounts of 1-amino-Δ2-1,2,4-triazolin-5-ones (13) or (17), Δ2- or Δ3-1,2,4-triazolin-5-ones (12), (18), or (19), and the s-triazine (20). A possible mechanism accounting for the products and the effects is discussed. A diradical intermediate and a hydrogen transfer from