The deterioration mechanism of reforming catalysts used for internal reforming molten carbonate fuel cells is studied through post-test analyses of the used catalysts. The catalysts studied are Ni/MgO-Al2O3. Deactivation of the catalysts is caused both by a decrease in Ni surface area of the catalyst and by loss of specific activity of the Ni surface (alkali poisoning). The decrease in Ni surface area is characterized firstly by accelerated sintering of Ni particles due to the existence of alkali metals (attached electrolyte). Secondly, sulfur poisoning also plays an important role, especially for sintered catalysts with small Ni surface areas. As for the effect of alkali poisoning, the specific activity of the Ni surface decreases to 15-60% of the original one, Blockage of pore structure by the electrolyte does not seem to he important for the loss of Ni surface area.