The synergistic inhibition effect between Cu2+ and 4-amino-5-methyl-4H-1,2,4-triazole-3-thiol (AMTT) on the corrosion behavior of mild steel in 0.5 M H2SO4, both in the absence and presence of chloride ions, has been investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The ptentiodynamic polarization and EIS results consistently revealed that Cu2+ and AMTT mixture had a synergistic inhibitive effect on the corrosion of mild steel in 0.5 M H2SO4 as a result of the cooperative adsorption of the two species on mild steel. AMTT alone and in combination with Cu2+ and/or chloride were found to obey Temkin adsorption isotherm. Chemical adsorption is proposed based on obtained values of G o ads.