The influence of iodoacetic acid (IAA) on the corrosion inhibition of mild steel in 0.5 M sulphuric acid (H2SO4) containing octylamine (OA) has been studied using weightloss, polarization and a.c. impedence techniques. Corrosion data obtained by different methods are consistent. IAA and OA individually retard the corrosion of mild steel in sulphuric acid. The addition of IAA enhances the inhibitor efficiency of OA considerably. Adsorption of inhibitors follows quasi-substitution process at the interfaces. OA cations are adsorbed by coulombic interaction on the metal surface, which is preoccupied by IAA molecules as dipoles. Adsorption model is suggested to account the synergistic action of IAA on the corrosion inhibition of mild steel in sulphuric acid solution by OA.