The self-assembled films of 1-methyl-5-mercapto-1,2,3,4-tetrazole (MMT) were prepared on the iron surface. By means of electrochemical impedance spectroscopy in 0.5 M H2SO4 solutions, the inhibition ability of the film was investigated. Results were discussed through changing the concentrations of the inhibitor and the pH values of the self-assembly solutions. Quantum chemical calculation was applied to elucidate the adsorption mechanism of the inhibitor molecule to iron atom. The study shows that MMT is a good inhibitor for iron in 0.5 M H2SO4 solutions. The self-assembled films formed in 10(-2) M acidic solutions have the best protection effect and the inhibition efficiency in 0.5 M H2SO4 solutions is 98.0%. Density functional theory proves that MMT molecule is adsorbed on the iron surface by the most negatively charged nitrogen atom and the adsorption can occur spontaneously. (C) 2007 Elsevier B.V. All rights reserved.