Molten carbonate fuel cell (MCFC) developers have considered replacing the standard 62/38 mol% Li/K with the 52/48 mol% Li/Na carbonate electrolyte. Here the oxygen dissolution behaviour in both melts was assessed indirectly, by studying the O-2 reduction electrochemically. It was inferred that diffusion limited responses in chronoamperometry measurements are a measure for the total oxygen solubility. A remarkable splitting was observed in the Arrhenius plot of these currents around roughly 650 degrees C for low pO(2) and pCO(2) values in the Li/K, but not in the Li/Na melt. This is explained as follows. In both melts only peroxide is present at high temperatures. In acidic Li/K melts at lower temperatures, additionally superoxide is stabilised, which species remains absent in the Li/Na melt. As a consequence the diffusion limited current in the Li/Na melt is at least a factor of 3 lower than in the Li/K melt at 650 degrees C in cathode gases lean in O-2. This factor increases dramatically with decreasing temperatures. Therefore large increases in the diffusion polarisation of the MCFC cathode with Li/Na are expected for cathode gases lean in oxygen, at temperatures below 650 degrees C.