Natural gas may be used instead of hydrogen as fuel for the molten carbonate fuel cell (MCFC) by steam reforming the natural gas inside the MCFC, using a nickel catalyst (internal reforming). The severe conditions inside the MCFC, however, require that the catalyst has a very high stability. In order to find suitable types of nickel catalysts and to obtain more knowledge about the deactivation mechanism(s) occurring during internal reforming, a series of nickel catalysts was prepared and subjected to stability tests at 973 K in an atmosphere containing steam and lithium and potassium hydroxide vapours. All the catalysts prepared showed a significant growth of the nickel crystallites during the test, especially one based on alpha-Al2O3 and a coprecipitated Ni/Al2O3 sample having a very high nickel content. However, this growth of nickel crystallites only partially explained the very strong deactivation observed in most cases. Only a coprecipitated nickel/alumina catalyst with high alumina content and a deposition-precipitation catalyst showed satisfactory residual activities. Addition of magnesium or lanthanum oxide to a coprecipitated nickel/alumina catalyst decreased the stability. Adsorption and retention of the alkali was the most important factor determining the stability of a catalyst in an atmosphere containing alkali hydroxides. This is because the catalyst bed may remain active if a small part of the catalyst bed retains all the alkali.