In this work, the performances of a spherical Co/gamma-Al2O3 eggshell catalyst characterized by a diameter of 600 mu m and an active shell region 75 mu m thick are assessed in a lab-scale fixed bed reactor operated at industrially relevant Fischer-Tropsch process conditions. Such performances are compared to those of a powder catalyst obtained by grinding and sieving a fraction of the eggshell catalyst so to obtain powders with a particle size distribution centered in the range 75-100 mu m. It is shown that, thanks to the presence of modest mass transport restrictions, the eggshell catalyst outperforms the corresponding powder in terms of C15+ specific yield. We explain these results by considering the effect of mass transport on the olefin re-adsorption probability and on the H-2/CO ratio in proximity to the active sites. The former is increased, and results in greater selectivity to heavy hydrocarbons. The latter is also increased, due to the negative order of CO conversion kinetics with respect to CO, and results in augmented reaction rate. (C) 2015 Elsevier B.V. All rights reserved.