The synergistic corrosion inhibition effect between chloride ion and the cationic gemini surfactant 1,3-propane-bis(dimethyl octodecyl ammonium bromide), designated 18-3-18, on cold rolled steel immersed in 0.5 mol L-1 sulfuric acid was investigated using gravimetric, polarization, and electrochemical impedance spectroscopy (EIS) methods. The polarization curve and EIS results were in agreement with the gravimetric measurements and demonstrated that the 18-3-18/Cl- system is an excellent inhibitor of steel corrosion in sulfuric acid. Estimates of thermodynamic parameters including adsorption Gibbs free energy (Delta G degrees(ads)), heat of adsorption (Delta H degrees(ads)), and entropy of adsorption (Delta S degrees(ads)) were obtained. The negative value for Gibbs free energy indicates that the adsorption process is spontaneous and the adsorbed layer on the steel surface is stable. The small positive value for the heat of adsorption and the small negative Gibbs free energy indicate that both chemical and physical adsorption of inhibitor molecules occurs simultaneously. A possible mechanism for the synergistic effect is also proposed based on the experimental results.