This paper reports on microkinetic measurements of cathodic oxygen reduction at immersed flag electrodes in Li/K carbonate melts and macrokinetic measurements of the same reaction in laboratory fuel cells with eutectic Li/K and Li/Na carbonate electrolyte. The microkinetic measurements confirm that at 650 degrees C oxygen reduction is a relatively fast reaction with exchange current densities of the order of 10(-2) A cm(-2) at gold, lithiated nickel oxide and lithium cobaltite electrodes, so that charge transfer hindrance might be of little relevance for the macrokinetics of molten carbonate fuel cell (MCFC) cathodes. O-2 reduction seems to be faster on Lithium cobaltate than on lithiated nickel oxide. The results on gold electrodes indicate that in Li/K carbonate melts the species which transports oxygen may be peroxocarbonate. Macrokinetic measurements show that mass transfer of oxygen determines current-voltage curves, in particular at low oxygen partial pressures but also at low carbon dioxide partial pressures of the order of 10(-1) bar and less, which are relevant to the technical operating conditions of MCFCs. CO2 transport bears sizeably on the cathodic current-voltage correlation at low partial pressures. Since the oxygen solubility is lower in Li/Na than in Li/K carbonate melts, oxygen overpotentials become much higher with lean gas mixtures at higher current densities with Li/Na carbonate melts. Therefore in the practical sense the Li/K carbonate melt seems to be overwhelmingly superior to the Li/Na carbonate eutectic in MCFC technology. If the morphology of MCFC cathodes can be significantly improved to remove oxygen and carbon dioxide mass transfer hindrance, or if pressurised cells are used in which O-2 and CO2 partial pressures exceed 0.25 bar even for lean gases, the less volatile and more conductive the Li/Na carbonate melt will be a superior alternative to the Li/K carbonate eutectic as an MCFC electrolyte.