Mono-acylated hydrazines are prepared in high yield by reaction of a hydrazine salt with an azole derivative of acyl halide in the presence of a strong base. Useful hydrazine salts include those of the formula RNHNH.sub.2 .multidot.HX, where R is alkyl, aryl, aralkyl, acyl, aroyl, heterocyclic or heteroaryl, and HX is a mineral or organic acid. Useful azole derivatives include those of the formula R'CO--A, where R' is alkyl, aryl, aralkyl, heterocyclic or heteroaryl and A is an azole radical such as pyrrole, imidazole, pyrazole, triazole, tetrazole, indole or benzimidazole.