Electrochemical measurements employing a.c. and d.c. techniques were used to study CO2 corrosion of carbon steel with two different microstructures: annealed, and quenched and tempered (Q&T), with and without inhibitors. The corrosive media was a deoxygenated 5% NaCl solution, saturated with CO2 at 40 degreesC and pH 6. Benzimidazole was employed as inhibitor, with a concentration of 100 ppm. Electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LRP) results showed that without inhibitor, Q&T samples have a better corrosion resistance than the annealed ones. On the other hand, the presence of inhibitor improves the corrosion resistance for the annealed samples whereas for the Q&T samples the opposite effect is observed. From the Bode phase angle plots it can be concluded that there is no evidence of inhibitor film formation in any microstructure condition. Based on the experimental findings in the present work, a mechanism of action for the inhibitor is proposed.