In recent years, polymer amines such as polyaniline has been reported as an efficient corrosion inhibitor for iron in acid media. In this paper, the performance of poly(diphenylamine) as corrosion inhibitor for iron in 0.5 M H2SO4 has been evaluated by potentiodynamic polarization, linear polarization and electrochemical impedance spectroscopy and compared with the performance of the diphenylamine monomer. It has been found that poly(diphenylamine) is an efficient inhibitor since the maximum efficiency of 96% has been observed at very low concentration of 10 ppm where as the monomer has given an efficiency of 75% at 1000 ppm. Besides, poly(diphenylamine) has been found to improve the passivation characteristics of iron in 0.5 M H2SO4. FTIR studies have shown that the poly(diphenylamine) is strongly adsorbed on the iron surface and inhibits the corrosion effectively. (c) 2005 Elsevier B.V. All rights reserved.