The corrosion and electrochemical behaviours of Li1-xNi1+xO2 and Li1-x(NiyCo1-y)(1+x)O-2 oxides were investigated in a molten carbonate electrolyte at 650 degrees C and compared with a NiO reference cathode material. These oxides are future candidate cathode materials for molten carbonate fuel cell (MCFC) and are divided in two families: monophasic oxides and biphasic oxides. The monophasic oxides show an important dissolution or a lower catalytic activity and are not good candidates for future use in MCFC. The biphasic oxides show a low dissolution and a good catalytic efficiency close to the NiO value. In this first study of new cathode materials by electrochemical impedance spectroscopy (EIS), it appears that the biphasic Li-Ni-Co-O oxides are the best candidates for MCFC. The MCFC electrochemical cathodic mechanism, taking into account the peroxide and the superoxide pathways and the O-2(-), CO2 and H2O diffusion proposed in the bibliography [I. Uchida, T. Nishina, Y. Mugikura, K. Itaya, J. Electroanal. Chem., 206 (1986) 229; C. Yuh, J.R. Selman, AIChE J., 34(12) (1988) 1949; T. Nishina, I. Uchida, Proc. Symp. Molten Carbonate Fuel Cell Technol., The Electrochem. Sec., PV90-16 (1990) 438; T. Nishina, G. Lindbergh, T. Kudo, I. Uchida, The International Fuel Cell Conference Proceedings, NEDO (1992) 189-192], is used to discuss the EIS results. The limiting rate of the peroxide or/and superoxide reactions of the new oxides was compared with the NiO reference oxide.