The synergistic effect between 4-amino-5-methyl-4H-1,2,4-triazole-3-thiol (AMTT) and halides on the corrosion of mild steel in 0.5 M H2SO4 solution at 20 oC is investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Experimental results revealed the significant synergistic action of halides on the protective effect of AMTT. Both protection efficiency (%P) and degree of surface coverage (θ) increase with increasing AMTT concentration, and the effect of halides on the protection efficiency follow the trend: Cl- < Br- < Iindicating that the radii and electronegativity of the halide play a prominent role in the adsorption process. In contrary the synergism followed the reverse order due to the effective inhibition efficiency of iodide. The adsorption of the AMTT accords with the Temkin adsorption isotherm. Some thermodynamic and kinetic parameters have been calculated and discussed.