The effect of water on electrode kinetics and mechanism of the oxygen reduction at the fully immersed flag-type Au electrode in (62 + 38)mol% (Li + K)CO3 melt at 650 degrees C have been investigated using the cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and the digital simulation of the single scan voltammograms. The cyclic voltammograms of the oxygen reduction under P-O2 : P-CO2 = 0.9:0.1 atm with 2.9-15.4 Torr H2O vapor showed more reversible waves than those obtained under a dry atmosphere. The EIS spectra at the open circuit potential for the oxygen reduction also showed distinctly reduced impedance values in the presence of water. From the analysis of both the data, it was found that the apparent mass transfer parameter C root D was increased by the presence of water, indicating that water can act as an acid species in the neutralization reaction of the oxide ion according to the reaction H2O + O2- reversible arrow 2OH(-).