Abstract: The cathodic reaction mechanisms in CO₂ corrosion of low-Cr steels were investigated by potentiodynamic polarization and galvanostatic measurements. Distinct but different dominant cathodic reactions were observed at different pH levels. At the higher pH level (pH >~5), H₂CO₃ reduction was the dominant cathodic reaction. The reaction was under activation control. At the lower pH level (pH <~3.5), H⁺ reduction became the dominant one and the reaction was under diffusion control. In the intermediate area, there was a transition region leading from one cathodic reaction to another. The measured electrochemical impedance spectrum corresponded to the proposed cathodic reaction mechanisms.