The influence of cobalt particle size in the range 3 to 18 nm on the Fischer-Tropsch synthesis intrinsic activity and product distribution was investigated over 26 alumina (gamma-Al2O3 and alpha-Al2O3) supported catalysts. A volcano-like curve was obtained for the gamma-Al2O3 based catalysts when the C5+ selectivity was plotted as a function of the particle size. The maximum selectivity was located at 7-8 nm. This is the first time an optimum particle size has been identified. An apparent optimum size was also discovered for the series of alpha-Al2O3 based catalysts. Interestingly, the C5+ selectivity of the alpha-Al2O3 based catalysts was higher than the selectivity of the gamma-Al2O3 based catalysts at all particle sizes. Thus, the selectivity is related both to the particle size and the support. No relation was observed between the cobalt particle size and the cobalt site-time yield. (C) 2008 Elsevier Inc. All rights reserved.