Hydrothermal conditions created by high conversion in Fischer-Tropsch hydrocarbon synthesis induce cobalt-silica mixed oxide formation causing deactivation of silica-supported cobalt catalysts. The mixed oxide phase displays a distinctive needle-like morphology with a 1-nm lattice spacing and has a Co/Si atomic ratio of 1:2/1. The hydrothermal origin of the mixed oxide is confirmed by the fact that steamed catalyst samples contain the same needle-like phase with identical lattice spacing and Co/Si ratio. Reduction in 1.2 MPa HZ at greater than or equal to 400 degreesC decomposes the mixed oxide and recovers small metallic cobalt particles. (C) 2003 Elsevier Science (USA). All rights reserved.