The corrosion inhibition of API 5L-X52 steel was studied by using an amido-imidazoline derivative (EN-901) and halide ions as an additive in CO2-saturated 3% NaCl solution with electrochemical techniques. The corrosion rate of the steel decreased with increase in EN-901 concentration while the inhibition efficiency increased and synergistically enhanced on addition of halide (Br- and I-) ions with the most profound effect obtained with iodide ion. The values of synergism parameter found to be greater than unity. The observed difference in behavior of both halide additives could be linked to the differences in their atomic radii as well as electronegativity. The experimental results revealed that the presence of halide ions in the solution stabilized the adsorption of imidazoline molecules on the steel surface thereby improving the inhibition efficiency of the inhibitor. The adsorption characteristics of the inhibitor were approximated by adsorption isotherms.