Zr promotion has been shown to improve the activity of Co/SiO2 and Co/Al2O3 Fischer-Tropsch synthesis (FTS) catalysts. However, little is known about how it works. The focus of this study was to investigate the impact of Zr modification of alumina used to support Co catalysts. Zr was first impregnated into alumina to produce Zr-modified alumina supports containing 2-11 wt% of ZrO2 in the final catalyst. Co catalysts having 20 wt% Co were then prepared from these supports by incipient wetness. It was found that Zr modification had a significant impact on the catalyst properties: FTS rate per gram of catalyst increased significantly (> 65% at steady state) with Zr modification and Co reducibility, especially during standard reduction, also increased. H-2 chemisorption, however, was found to be essentially constant with Zr modification. The impact of Zr modification is likely due to stabilization of the alumina support, prevention or blockage of Co surface "aluminate" formation, and an increase in Co reducibility to the active catalytic metallic phase. SSITKA results for CO hydrogenation showed an increase in the number of active surface intermediates (N-M) with Zr modification while the intrinsic activity (1/tau(M)) remained constant, confirming that the major impact of Zr was in increasing the concentration of active Co surface sites. Because of the inconsistency of the turnover frequency calculated based on H-2 chemisorption with the 1/tau(M) results, it is suggested that the standard adsorption conditions usually used for Co may not be adequate for modified Co catalysts such as the ones studied here. (C) 2003 Elsevier Science (USA). All rights reserved.