The electrode kinetics and mechanism of oxygen reduction at gold electrodes in (53 + 47)mol% (Li + Na)CO3 melt at 923 K under pressurized conditions have been studied to elucidate the effect of pressure on molten carbonate fuel cell (MCFC) performance. The partial pressure of oxygen, P-O2, was changed from 0.1 to 3.6 atm with a constant P-CO2 fixed at 0.9 atm, and vice versa. From cyclic voltammetric measurements and ac impedance measurements, it was found that the reaction was highly reversible and diffusion controlled. The reaction order analysis of Warburg coefficients sigma(app), which were obtained by ac impedance measurements, revealed that the reaction process is dominated by the mixed diffusion of superoxide ion O-2(-) and dissolved CO2. Then, sigma(O2-) and sigma(CO2) were separated from the apparent sigma(app) value using the gas pressure dependence of O-2(-) and CO2 concentrations in the melt. The 1/sigma(O2-) values, which were proportional to the CD1/2 value of O-2(-) were found to be smaller than 1/sigma(CO2) values at 3.6 greater than or equal to P-O2 greater than or equal to 0.1 with a constant P-CO2 of 0.9 atm and at 3.6 greater than or equal to P-O2 greater than or equal to 0.9 with a constant P-O2 of 0.9 atm. It was concluded that the diffusion of O-2(-) is the controlling factor in the overall mass transfer process. Also, it was realized that the concentrations of O-2(-) and CO2 are about half of those in (62 + 38)mol% (Li + K)CO3 melt.