The reaction of carboxylic acid hydrazides, carbon disulfide and alcoholic alkali to form 2- substituted-A2-1, 3, 4-oxadiazo-line-5-thiones has been found to be general for aliphatic, aromatic and heterocyclic carboxylic acid hydrazides and dihydrazides. An interesting thermal isomerization of 2-(3-(5)-pyrazolyl)-A2-l, 3, 1-oxadiazoline-5-thione(i'111) to 7- mercaptopyrazolo [1, 5-d] as-triazin-4 (5H)-one (X) was observed.