A Co/SiO2 catalyst recovered after both gas and liquid phase Fischer-Tropsch synthesis exhibited significant decreases in the amount of reducible metal. Hydrothermal conditions similar to those occurring during Fischer-Tropsch synthesis were simulated in order to study compound formation in this Co/SiO2 catalyst. Hydrothermal treatment at 220 degrees C led to a catalyst with lower reducibility, attributable to the formation of both reducible and nonreducible (<900 degrees C) Co silicates. The formation of these compounds occurred only when metallic Co was present, was more pronounced in the presence of hydrogen, and was inhibited by air.