This paper addresses the effect of catalyst particle size distributions on Fischer-Tropsch catalytic performance for cobalt catalysts promoted by rhenium on alumina or modified alumina as catalyst support. The catalyst particle size will affect diffusion of reactants as well as of products. Some key findings are that the C5+ selectivity is rather constant for particle sizes up to ca. 400 mu m and that the selectivity reduction thereafter is largely explained by an increase in the methane yield as the effective H-2/CO ratio increases. In the whole range of particle sizes investigated, the olefin to paraffin ratio of the light products decreases linearly as the diffusion length becomes longer. Moreover, this ratio is the same for all catalysts in spite of a large variation in selectivity to higher hydrocarbons. It follows that readsorption of olefins and continued chain growth only plays a minor role. The roles of added water and conversion level are also discussed.