Some new fused heterocyclic systems such as 1,2,4-triazolo[3,4-b]1,3,4-thiadiazoles and 1,3,4-thiadiazines were synthesized through the reaction of the starting material 4-(4-amino-5-mercapto-4H-1,2,4-triazol-3-yl)phthalazin-1(2H)-one with triethyl orthoformate, furoyl chloride, thiophene-2-aldehyde, phenylisothiocyanate, chloroacetonitrile, and p-nitro-2-bromoacetophenone.