The electrochemical behavior of carbon steel in high pressure CO2-containing formation water was studied by in situ electrochemical measurements in an autoclave. According to a thermodynamics model, the equilibrium concentrations of various species and the equilibrium potentials of various electrode reactions were calculated to determine the possible electrode reactions. Electrochemical measurements show that in the initial stage, the impedance spectra exhibit three time constants, i.e., capacitive semicircle at high frequency, inductive semicircle at intermediate frequency and capacitive semicircle at low frequency. As corrosion proceeds, the inductive semicircle shrinks and the capacitive semicircle at low frequency enlarges. Impedance spectra exhibit double capacitive semicircles later. In the last stage, Warburg impedance representing diffusion effect appears at low frequency. The electrochemical mechanism is closely related to the formation process of corrosion scales. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.030209jes] All rights reserved.