Polyaniline and metal cations such as Zn2+ and Mn2+ ions have been found to inhibit corrosion of iron in sulphuric acid moderately. It has been reported that the metal cations are found to interact directly with polyaniline via protonation of imine nitrogen atom of quinoid groups of polyaniline and induce chemical and conformational changes to polymer. Hence a study has been made on the inhibition of iron in 0.5 M H2SO4 by polyaniline in the presence of Zn2+ and Mn2+ ions (1 x 10(-3) M) by electrochemical impedance spectroscopy, linear polarization and Tafel polarization methods. The inhibition efficiencies of polyaniline are found to be 40% for 10 ppm and 72% for 100 ppm. But in the presence of metal cations, the inhibition efficiencies of polyaniline are found to be increased to 80% for 10 ppm and more than 90% for 100 ppm. Higher inhibition efficiency of polyaniline in the presence of metal cation is attributed to the increased electron rich benzenoid groups absorb more effectively on iron surface and there by reduces the corrosion rate markedly. (c) 2007 Elsevier B.V. All rights reserved.