Cathodic reactions, specially the oxygen and hydrogen activities, have been extensively investigated on carbon steel in simulated fuel-grade ethanol (SFGE) environments using cathodic potentiodynamic polarization and electrochemical impedance spectroscopy. Under normal conditions, oxygen reduction is the major cathodic reaction for carbon steel in SFGE. Hydrogen evolution, confirmed by hydrogen permeation test, is activated by de-aeration and at much higher cathodic overpotential. The dissociation of acetic acid in SFGE enhances the proton related cathodic reactions. Water in ethanol can increase the proton dissociation rate, resulting in a higher cathodic current density. At potentials close to free corrosion potential, the reduction of surface oxidized products contributes to the cathodic activities in SFGE. Based on this study, the possible effects of cathodic reactions on corrosion of carbon steel in SFGE are also discussed. (C) 2010 Elsevier Ltd. All rights reserved.