The effect of different modifications (Zn, Mg, Ni, Re) has been studied for alumina-supported 12 wt% Co-catalysts in the Fischer-Tropsch synthesis (FTS) using a fixed-bed reactor at 483 K, 20 bar, and H(2)/CO = 2. Different parameters including calcination temperature, loading, impregnation sequence, and water partial pressure during FTS have been studied. When compared to low surface area alpha-Al(2)O(3), mechanical strength was substantially improved for Ni- and Mg-modified aluminas calcined at very high temperatures (>1400 K), thus making them more suitable for slurry or fluidized-bed operation. However, Mg was found to have a loading-dependent negative effect on both activity and selectivity. The wt% effect was stronger when co-impregnated with Co-Re and calcined at 773 K, than when impregnated on the support and calcined at a high temperature (1173 K) prior to impregnation with Co-Re. Co-impregnating Co-Re with Zn also had a strong loading-dependent negative effect on activity and selectivity, while impregnating Zn on the support and calcining at a high temperature (1173 K) prior to impregnation with Co-Re had no negative effect on the overall C(5+) yield. The negative effects of Mg and Zn could not be explained by dispersion or particle size effects and were likely related to a chemical/site effect similar to that of alkalies reported on in the literature. The effect of water for the Ni-modified support was in accordance with the literature, improving reaction rates and C(5+), selectivity, while inhibiting olefin hydrogenation, as demonstrated by the propene/propane ratio. The catalysts were characterized with H(2) chemisorption, N(2) sorption, mercury intrusion, X-ray diffraction, temperature-programmed reduction, and O(2) titration. (C) 2011 Elsevier Inc. All rights reserved.