Square channel cordierite monoliths have been loaded with alumina washcoat layers of various thicknesses (20-110 mu m) and loaded with rhenium and cobalt resulting in a 0.1 wt.% Re/17 wt.% Co/Al2O3 catalyst. These monolithic catalysts have been tested in the Fischer-Tropsch synthesis in a temperature window (180-225 degrees C) under synthesis gas compositions ranging from stoichiometrically excess carbon monoxide to excess hydrogen (H-2/CO = 1-3). The results include data on the activity and selectivity of CoRe/Al2O3 monolithic catalysts for FTS under these process conditions. Washcoat layers thicker than about 50 mu m appear to lead to internal diffusion limitations. Thinner washcoat layers yield, depending on the conditions, to larger amounts of alpha-olefins than alkanes for chain lengths below 10 carbon atoms. ASF and non-ASF chain length distributions are obtained for thin washcoats, whereby the chain growth probability increases from 0.83 to 0.93. Under certain conditions the amounts of alkanes even increase with chain length. These experimental results with different diffusion lengths have been used to analyze the effects of secondary reactions on FTS selectivity. (c) 2005 Elsevier B.V. All rights reserved.