Three supported catalysts containing 20 wt% cobalt and 0.5 wt% rhenium were subjected to electron microscopy studies in their calcined state. The catalysts were prepared by incipient wetness impregnation of gamma-Al2O3 supports of different pore characteristics with aqueous solutions of cobalt nitrate hexahydrate and perrhenic acid. The influence of the support on the Co3O4 crystallite size and distribution was studied by X-ray diffraction and electron microscopy. There was a positive correlation between the pore diameter of the support and the post calcination Co3O4 crystallite size. On all three gamma-Al2O3 supports, Co3O4 was present as aggregates of many crystallites (20-270 nm in size). Cobalt oxide did not crystallise as independent crystallites, but as an interconnected network, with a roughly common crystallographic orientation, within the matrix pore structure. The internal variations in crystallite size between the catalysts were maintained after reduction. Fischer-Tropsch synthesis was carried out in a fixed-bed reactor at industrial conditions (T = 483 K, P = 20 bar, H-2/CO = 2.1). Although the cobalt-time yields varied significantly (4.6-6.7 x 10-3 mol CO/mol Co s), the site-time yields were constant (63-68 x 10-3 s-1) for the three samples. The C5+ selectivity could not be correlated to the cobalt oxide aggregate size and is more likely related to the cobalt particle size and chemical properties of the gamma-Al2O3 support.