Li-Ni oxide mixtures with high lithium content are considered to be an alternative cathode material for molten carbonate fuel cells (MCFCs). The electrochemical behaviour of Li0.4Ni0.6O samples has been investigated in a Li-K carbonate melt at 650 degreesC by electrochemical impedance spectroscopy as a function of immersion time and O-2 and CO2 partial pressure. The impedance spectra have been interpreted using a transmission line model that includes contact impedance between reactive particles. The Li0.4Ni0.6O powder particles show structural changes due to high lithium leakage and low nickel dissolution from the reactive surface to the electrolyte during the first 100 h of immersion. After this time, the structure seems to be stable. The partial pressures of O-2 and CO2 affect the processes of oxygen reduction and Li-Ni oxide oxidation. X-ray diffraction and chemical analysis performed on samples before and after the electrochemical tests have confirmed that the lithium content decreases. SEM observations reveal a reduction in grain size after the electrochemical tests.