The synergistic inhibition between 4-(2-pyridylazo) resorcin (PAR) and chloride ion on the corrosion of cold rolled steel in 1.0 M phosphoric acid was studied using weight loss and potentiodynamic polarization method. Results obtained revealed that single PAR is not an effective inhibitor for steel corrosion in phosphoric acid, but in the presence of chloride ion, PAR may act as a good inhibitor due to the synergism. It is found that the adsorption of PAR accords with the Langmuir adsorption isotherm in the absence and presence of chloride ion. Potentiodynamic polarization studies show that PAR is an anodic inhibitor for steel in 1.0 M phosphoric acid, and with addition of chloride ion PAR acts as a mixed type inhibitor. The experimental temperature ranges from 30 to 45 degrees C. The kinetic data such as apparent activation energies and pre-exponential factors at different concentrations of the inhibitor were calculated, and the effect of the apparent activation energies and pre-exponential factors on the corrosion rates of cold rolled steel was discussed. The inhibitive action was satisfactorily explained by using kinetic models. (c) 2006 Elsevier B.V. All rights reserved.