Film-forming corrosion inhibitors are extensively used to control CO2 corrosion of steel in oil and gas production plants. The performance of these inhibitors is connected to the persistence and/or regeneration of the protective film. Their active mechanism in the presence of CO2 is still little understood as far as the kinetics is concerned. In this paper, some of the factors affecting this mechanism have been studied with the aid of polarization curves and electrochemical impedance, using rotating disc and cylinder electrodes. The performance of a commercially available film-forming inhibitor used in PETROBRAS, the Brazilian petroleum company, was evaluated. The results show that the efficiency of the inhibitor depends on its residual concentration and on the hydrodynamic flow conditions. The impedance diagrams provided information on growth, stability and deterioration of the inhibitor film.