The effect of iodide ions on inhibitive performance of 1,4,6-trimethy1-2-oxo-1,2-dihydropyridine-3carbonitrile (TODPCN) on mild steel (MS) corrosion in 0.5 M H2SO4 was studied using gravimetric and electrochemical measurements. TODPCN inhibits the corrosion of MS to the extent of 62.3% at its lowest concentration (0.5 mM) and its inhibition efficiency (eta) further increases on increasing concentration at 298 K. The adsorption of TODPCN on MS was found to follow the Langmuir adsorption isotherm. The value of a? increased on the addition of 2.0 mM KI The value of synergism parameter being more than unity indicates that the enhanced a? value in the presence of iodide ions is only due to synergism. Thus, a cooperative mechanism of inhibition exists between the iodide anion and TODPCN cations. The increase in surface coverage in the presence of KI indicates that iodide ions enhance the adsorption of TODPCN. The surface morphology of corroded/inhibited MS was studied by atomic force microscopy. X-ray photoelectron spectroscopy of inhibited MS surface was carried out to determine the composition of the adsorbed film. Some quantum chemical parameters and the Mulliken charge densities for TODPCN calculated by density functional theory provided further insight into the mechanism of inhibition. (C) 2016 Elsevier B.V. All rights reserved.