The inhibition of the corrosion of N80 mild steel by 2-undecyl-1-1-ethylamino-1-ethylcarboxyl quaternary imidazoline (CQI) in CO2-saturated 3% NaCl and 3% Na2SO4 solutions was studied using electrochemical methods. Inhibition efficiency increased with increase in CQI concentration and synergistically increased in the presence of halide ions. CQI inhibited the corrosion reaction by chemical adsorption on the metal/solution interface in halide free solutions and by a combination of chemical and coulombic attraction in the presence of halide ions. The adsorption characteristics of the inhibitor were approximated by Frumkin isotherm and El-Awady et al. kinetic-thermodynamic model.