Both low loaded 15% Co/Al2O3 and more highly loaded 25% Co/Al2O3 catalysts are studied, in order to explore the impact of cluster size on the stability of the cobalt cluster to support-influenced reoxidation processes at high H2O/CO ratios. XAFS and activity data suggest that there are two regions for the water effect: at lower H2O/CO ratios water influences CO conversion by reversible kinetic effects while at higher H2O/CO ratios cobalt re-oxidation processes occur. The latter regime where water was added at and above 25% are examined. Synthesis conditions were maintained constant while argon balancing gas was replaced by added water. Catalyst samples were withdrawn from the reactor during synthesis at different partial pressures of added water and cooled in the wax product under inert gas. The EXAFS results suggest that, unlike the smaller clusters on unpromoted and, especially noble-metal promoted, 15% Co/Al2O3 catalysts, the larger crystallites (>10 nm by chemisorption and XRD) on 25% Co/Al2O3 undergo oxidation by H2O to CoO, most likely confined to the surface. The clusters are re-reduced when H2O was switched off, and the activity displayed an important recovery. (C) 2004 Elsevier B.V. All rights reserved.