A systematic study of the effect of gamma-Al2O3 Support variables on Fischer-Tropsch synthesis activity and selectivity was carried out at industrially relevant conditions (T = 483 K, P = 20 bar, H-2/CO = 2. 1). A total of 13 catalysts were prepared by incipient wetness impregnation of gamma-Al2O3 supports of varying pore characteristics and chemical purities with aqueous solutions containing the required amounts of cobalt and rhenium precursor to give nominal loadings of 20 and 0.5 wt%, respectively. The catalysts were analysed for cobalt, rhenium, nitrogen, and sodium and characterised by nitrogen adsorption/desorption, mercury intrusion, X-ray diffraction, hydrogen chemisorption, temperature-programmed reduction, and oxygen titration. The size of the Co3O4 cobalt particles was controlled by the support pore size, with small particles formed in narrow pores and large particles formed in wide pores. The degree of reduction increased with increasing catalyst pore size and Co3O4 particle size. The catalysts contained different amounts of sodium (20-113 ppm) and the site-time yield decreased with increasing sodium content. Positive correlations were found between cobalt particle size and C5+ selectivity and between catalyst pore size and C5+ selectivity. The results indicate that the C5+ selectivity is controlled by the size and appearance of cobalt particles. (C) 2007 Elsevier Inc. All rights reserved.