Cobalt based Fischer-Tropsch synthesis catalysts were studied by adsorption microcalorimetry. A series of catalysts consisting of 20 wt% cobalt supported on gamma-Al2O3 were examined. Cobalt particle size as evaluated by H-2 chemisorption varied from 4.8 to 16.7 nm in diameter. H-2 and CO were examined as probe molecules and their adsorption on reduced cobalt catalyst was investigated. The results show only minor differences in the initial heat of adsorption for the studied catalysts, while at intermediate coverages (20-60%) the heat of adsorption appears to be nearly identical. (C) 2013 Elsevier B.V. All rights reserved.