Improvement of the molten carbonate fuel cell electrolyte is a key parameter to increase the performance of such electrochemical generator. One of the main challenges is to enhance the global oxygen reduction at the state-of-the-art porous nickel cathode. In this study, the effect of adding 5 mol% of caesium in Li-K and Li-Na molten carbonate eutectics or 5 mol% of rubidium in Li-K melt was analysed with respect to the behaviour of nickel cathode towards oxygen reduction. Evolution of the open-circuit potential and electrochemical impedance spectroscopy measurements were carried out over time. In Li- K melt, both Cs and Rb additives induced an improved cathode behaviour: more rapid equilibration reaching more rapidly the equilibrium potential, and significantly lower total resistance (9 times less with Cs and 3 times less with Rb), in particular mass transport, with respect to the pristine electrolyte. Moreover, charge transfer resistance was significantly lower with Rb and nearly the same with Cs versus pristine Li-K. Both additions are significantly positive for enhancing oxygen reduction at the porous electrode, which seems to be particularly the case for Cs addition. However, addition of Cs to Li-Na did not show an important effect. Changing the composition of Li-K with the mentioned additives could be an important step towards a more performing MCFC, but more insight on oxygen reduction kinetics with Rb addition and single cell tests with both cases are compulsory. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.