Abstract 3, 4-Disubstituted-1, 2, 4-thiadiazole-5 (4H)-thiones (4) have been prepared by the treatment of 3, 4-di-substituted-1, 2, 4-thiadiazole-5 (4H)-ones (3) with P4S10. S-Methyl derivatives were obtained from the methylation of 3-substituted-1, 2, 4-thiadiazole-5 (4H)- thiones with CH2N2.