This study attempts to characterize the gas-phase reactions at the anode of a molten carbonate fuel cell by the addition of carbon monoxide and carbon dioxide. Carbon dioxide addition causes a rapid negative voltage shift. A constant voltage is maintained during the addition at open-circuit state. This indicates that the water gas shift (wgs) reaction which accompanies the addition of carbon dioxide is a fast reaction. Addition of carbon monoxide results in slow shifts in negative and positive voltages and suggests that carbon monoxide gives rise to complex gas-phase reactions at the anode. The Boudouard reaction prevails at the anode on addition of carbon monoxide at low partial pressures of carbon dioxide, i.e. H-2:CO2 = 90:10 to 80:20 mol%, while the wgs reaction is dominant at higher carbon dioxide ratios. In a polarized state, the wgs reaction appears to be dominant, as opposed to the Boudouard reaction, even at a low CO2 Partial pressure of H-2:CO2 = 90:10 mol%. This is probably due to an increase in the partial pressure of carbon dioxide caused by the anodic reactions. (C) 2003 Elsevier B.V. All rights reserved.