Phenyl-, p-chlorophenyl-, and 3,4-methylenedioxyphenyl-propiolic acids react with hydrazine to give, in each case, a mixture of the corresponding pyrazol-5-one and azine. p-Methoxyphenyl-, and 3,4-dimethoxyphenyl-propiolic acids give azines only. Phenyl-, p-chlorophenyl-, and m-chlorophenyl-propiolic acids react with phenylhydrazine to give the corresponding 5-aryl-1-phenylpyrazol-3-ones.