2-[4-(Methylthio) phenyl] acetohydrazide (HYD) and 5[4-(methylthio)benzyl]-4H-1,2,4-triazole-3-thiol (TRD) are compounds that contain methylthiophenyl moiety and substantially inhibit corrosion of zinc in acidic medium. The relationship between molecular structures and inhibition efficiencies of these intriguing inhibitors remains the subject of intense speculation, as introduction of thiophenyl group in strategic positions of many molecules alters their activity. We have performed density functional theory calculations at the B3LYP/6-311++G(d,p) level to analyze the inhibition mechanisms proposed for these compounds. In agreement with experiments, the results afford a full explanation of the highest efficiency observed for TRD in terms of electronic and structural characteristics.