A series of mesoporous silica supported cobalt catalysts for the Fischer-Tropsch reaction with pore diameters from 3 to 22 urn was synthesized and characterized. Detailed X-ray diffraction measurements were used to determine the composition and particle diameters of the metal fraction, analyzed as a three-phase system containing Co-fcc, Co-hcp, and CoO. Catalyst properties were determined at three stages of catalyst history: (1) after the initial calcination step to thermally decompose the catalyst precursor into Co3O4, (2) after the hydrogen reduction step to activate the catalyst to Co and (3) after the FT reaction. Small Co-hcp particles were detected in all reduced catalysts contributing significantly to the Co surface area. The reaction rate increased with the pore diameter of the silica support. The results indicate the importance of careful catalyst characterization in determining die factors that contribute to reactivity. In addition, the identification of significant quantities of two cobalt metal phases suggests that further study of the intrinsic activity of each phase, as well as the structural features of the supports that determine the distribution of cobalt phases and particle sizes is warranted. (C) 2010 Elsevier B.V. All rights reserved.