Microwave irradiation of 1-substituted-2, 5-dithiobiureas and 1, 6-disubstituted-2, 5-dithiobiureas with dimethyl ethynedicarboxylate gave methyl 2-6-oxo-2-(substituted amino)imidazo-[2, 1- b][1, 3, 5]thiadiazol-5(6 H)-ylidene} acetate and methyl 7-oxo-1, 3-bis(substituted imino)-3, 7-dihydro-1 H-pyrazolo[1, 2- c][1, 3, 4]thiadiazole-5-carboxylate. Rationales for these transformations are presented.