The electrode kinetics and mechanism of oxygen reduction at gold electrodes in (62 + 38)mo1% (Li + K)CO 3 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, Po2, was changed from 0.1 to 3.6atm with a constant Pco2 fixed at 0.9atm, and vice versa. Cyclic voltammetric measurements showed that the reaction was highly reversible, and the current density increased with increasing Po2, indicating that the concentration of the reactive species for 02 reduction increased. To clarify the oxygen reduction path, reaction order analysis of Warburg coefficients (~r), which were obtained by ac impedance measurements, has been carried out. It was found that the reaction process is dominated by the mixed diffusion of superoxide ions, 02 and CO 2 . Therefore, ao_ and ~rco ~ were separated from the apparent a value, aapp, using the pressure dependence of 02 and CO 2 concentrations in the melt. The 1/ao~ values, which were proportional to the CD 1/2 value of 02 were found to be smaller than 1/aco 2 values at 3.6 1> Po2 ~> 0.l with a constant Pco~ of 0.9 atm and at 3.6 1> Pco~ I> 0.9 with a constant Po2 of 0.9 atm. It was concluded that the diffusion of 02 is the controlling factor in the overall mass transfer process. Based on a relationship between aap p and partial pressures of the oxidant gas, the optimum cathode gas composition of air and CO 2 mixtures was estimated as a function of total gas pressure, to minimize the diffusion impedance.