The effect of O2 and CO2 gas partial pressures on the O2 reduction path at a gold flag electrode has been investigated in (35 + 65) mol percent (m/o) (Li + K)CO3, (62 + 38) m/o (Li + K)CO3, (75 + 25) m/o (Li + K)CO3, (52 + 48) m/o (Li + Na)CO3, and (48.5 + 31.5 + 25.0) m/o (Li + Na + K)CO3 melts at 615 to 800-degrees-C. We carried out precise measurements of the Warburg coefficient of the ac impedance at the rest potential, using a Randles-Ershler equivalent circuit. Taking into account four possible diffusing species, i.e., O2-, O-2(2-), CO42-, and CO2, a graphical reaction order analysis was made using the Warburg coefficients. It was found that only the case of mixed diffusion of O2- and CO2 results in a satisfactory linear relationship. Thus, we conclude that the simultaneous diffusion of O2- and CO2 is the dominant feature of the oxygen reduction path in the molten carbonate mixtures used in this study. Based on this conclusion, we estimated the diffusion coefficient of O2- and CO2.