Calcined, reduced and spent Co-Re/γ-Al2O3 catalysts for Fischer–Tropsch synthesis (FTS) were characterized and compared. Co-K-edge and Re-LIII-edge X-ray absorption near edge structure (XANES), X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and H2 chemisorption were used to provide insight into structural transformations that the catalyst experiences during a month of operation in a semi-commercial FTS plant. Results from the core techniques for nanoparticle size determination suggest that sintering of the cobalt crystallites is an important deactivation mechanism in FTS performed in slurry reactors. In addition, a higher degree of reduction is observed for the spent catalytic material, while Re appears in a partially reduced state before and after reaction. The particle size distribution together with the spread of Co nanoparticles on the γ-Al2O3 surface indicates crystallite migration as the prevailing mechanism.