The copolymer poly (aniline-co-o-toluidine) noted poly(ANI-co-OT) is chemically synthesized, and characterized by MIR, UV-Vis, TGA and XRD techniques. The results show that this copolymer presents new structures and it is more thermally stable than the corresponding parent homopolymers. It is found that it is soluble in dimethylformamide (DMF), while the homopolymers have a low solubility. Techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) have been employed to study the inhibition effects of this copolymer on the corrosion of carbon steel in 3% NaCl solution. The results show an increase in inhibition efficiency as a function of copolymer concentration to attain an optimum (70%) for a concentration close to 100 ppm. This is due to the higher coverage of the carbon steel electrode surface as shown by the Temkin adsorption isotherm. The Potentiodynamic polarisation studies reveal that this copolymer acts as a mixed-type inhibitor. The results obtained by potentiodynamic and EIS techniques are in good agreement. (C) 2009 Elsevier B.V. All rights reserved.