In molten carbonate fuel cells (MCFC), the wettability of the electrode and the electrolyte distribution are very important factors influencing the active reaction area. We have observed the molten carbonate behaviour directly on the cathode (porous NiO) and the electrolyte plate (LiAlO2) under various gas conditions and at controlled potentials using an environmental scanning electron microscope (ESEM) equipped with a hot stage. We estimated the liquid electrolyte distribution in the cathode and measured the contact angles on NiO and LiAlO2 in the electrolyte. Moreover, the electrolyte movement in the reaction CO2 + 1/2 O-2 + 2e(-) = CO32- was observed on the surface of the porous NiO in a CO2/O-2 atmosphere. The reaction CO32- + 2e(-) = CO + 2 O-2- of the gas generation was observed in a H2O atmosphere. The active reaction points on the electrode are the areas where the electrolyte film is thin.