Different alumina phases i.e. delta-, theta-, and alpha-Al2O3 were prepared from two different samples of gamma-Al2O3 by heat treatment. The alumina samples were categorised as small and medium pore according to the pore size distribution. Nominally 12 wt% cobalt was loaded onto the supports by incipient wetness impregnation. By varying the amount of deionised water and ethylene glycol, the Co3O4 crystallite size could be controlled. After reduction, a relatively constant cobalt metal particle size of 8-10 nm was obtained. Temperature-programmed reduction profiles showed that the medium pore catalysts were reduced at lower temperatures as compared to the small pore catalysts. The Fischer-Tropsch synthesis was carried out for all samples under the same experimental conditions (P=20 bar, T=483 K, H-2/CO=2.1). The medium pore catalysts exhibited higher selectivity to C5+, than the small pore catalysts. Furthermore, higher C5+, selectivity was observed for delta-and alpha-Al2O3 samples compared to gamma-and theta-Al2O3, with alpha-Al2O3 giving the highest selectivity. Steady-state isotopic transient kinetic analysis (SSITKA) showed that the amount of CHx surface intermediates followed the same trend as the C5+ selectivity with respect to pore size and alumina phase. (C) 2010 Elsevier B.V. All rights reserved.