The synergistic effect of halide ions on the inhibition of corrosion of iron in 0.5 M H2SO4 solutions by polyaniline has been studied by polarization and impedance methods. Addition of 0.5 x 10(-3) M I- ions enhanced the inhibition efficiency of polyaniline at 10 ppm from 53% to 90%. However in the case of bromide and chloride ions, the inhibition efficiency of polyaniline at 50 ppm has been increased from 71% to 90%. The synergism parameter in all cases is found to be greater than 1. The synergistic effect is attributed to enhanced adsorption of polyaniline by the adsorbed halide ions. (c) 2005 Elsevier B.V. All rights reserved.