The inhibition efficiency of an imidazoline derivative for APIX65 steel in CO2-saturated 5% NaCl solution has been studied by using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. This imidazoline derivative acts as a mixed-type inhibitor, which suppresses both cathodic and anodic processes by its adsorption on the electrode surface according to Langmuir adsorption isotherm, together with a slight positive shift in corrosion potential (E-corr). The adsorbed inhibitor on the surface of electrode affects the kinetic processes of cathodic and anodic reactions and increases the reaction activation energy. Both potentiodynamic polarization and EIS measurements reveal that this imidazoline derivative inhibits the corrosion of APIX65 steel in CO2-saturated 5% NaCl solution and that the inhibition efficiency increases with increasing of the inhibitor concentration. The Nyquist plot of uninhibited electrode at E-corr is characterized by a depressed semicircle from high to medium frequencies and an inductive loop at low frequencies while the inhibited ones emerge "degradation" phenomena with simple depressed capacitive loops. The semicircles representing the inhibitor film merge with the charge transfer loops. The inhibitor films form gradually and the inhibition efficiency increases with increase of immersion time. (c) 2007 Elsevier B.V. All rights reserved.