A cobalt based Fischer-Tropsch catalyst was studied during the initial stages of the reaction at industrially relevant conditions. The catalyst consists of 20 wt% cobalt supported on gamma-Al2O3 and promoted by 1 wt% of rhenium. X-ray diffraction (XRD) in combination with X-ray absorption near edge structure (XANES) were used to reveal information on the particle and crystallite size and the oxidation states of the active component. Conditions giving high selectivity towards light hydrocarbons (methanation, 673 K, 10 bar and high GHSV) were compared to conditions were higher hydrocarbons are the main products (FT synthesis at 483 K, 18 bar and low GHSV). The data analysis shows no significant changes in the cobalt crystallites during the first hours of Fischer-Tropsch synthesis. Running the reaction at higher temperatures and predominantly methanation conditions led to significant sintering of the cobalt particles and a further reduction of a partially reduced catalyst could be observed. (C) 2009 Elsevier B.V. All rights reserved.