Co/SBA-15 (30 wt.%) catalysts with different pore sizes were prepared from the same materials by incipient wetness impregnation and characterized by diffuse reflectance infrared Fourier transform spectroscopy, N-2 adsorption-desorption, X-ray diffractometry, temperature-programmed reduction and H-2 desorption, oxygen titration as well as transmission electron microscopy. The reduction of the catalysts took place in two stages with CO3O4 reduction to CoO and then to Co-0. The first stage reduction was facile regardless of the catalyst Pore size while the second stage reduction was much easier on the catalysts with larger pore. After the reduction, cobalt particles were found to be distributed on both the exterior and interior surfaces of the support. Compared to the catalyst with smaller pores, larger pore catalysts have more adsorption sites for CO of both the linear and bridge types, The catalysts with larger pore led to larger cobalt cluster size, lower dispersion and higher reducibility. The larger pores gave rise to more adsorbed CO of the bridge-type on FTS. CO conversion increased and then decreased with the pore size in the range studied. The catalysts with larger cobalt cluster size showed higher C5+ selectivity for the FTS.. (c) 2008 Elsevier B.V. All rights reserved.