The regioselectivity on a cyclic thioamide group towards different electrophiles was studied on the model compound 4-methyl-1-thioxo-1,2,4,5-tetrahydro[1,2,4]triazolo [4,3-a]quinazolin-5-one 1. The examined compound 1 reacts with alkyl halides, amines in the presence of formaldehyde, acyl halides and compounds having activated double bonds to afford the N-substituted derivatives 2, 3 and 6. The regioselective reactions on nitrogen atom are due to strong Coulombic attraction. The reaction of 1 with amines in the presence of hydrogen peroxide afforded the aminolysis product 4. Compounds 1-6 were identified by FTIR, 1H NMR, 13C NMR, and mass spectroscopy.