The effect of water on Co/Al2O3 and CoRe/Al2O3 catalysts has been studied by adding water to the synthesis gas feed and by model studies exposing the catalysts to H2O/H-2 feeds using several characterization techniques such as TPR, gravimetry, XPS, TPD and pulse adsorption. It was found that the CoRe/Al2O3 catalyst deactivates when water is added during Fischer-Tropsch synthesis and model-studies showed that this catalyst oxidizes in H2O/H-2 mixtures with a ratio much lower than expected for oxidation of bulk cobalt. The reoxidation increases with increasing H2O partial pressure and H2O/H-2 ratio. TPR and gravimetry showed only small amounts of bulk reoxidation, while pulse adsorption and TPD indicated large decreases in Co-surface metal. It is suggested that oxidation of highly dispersed phases or surface oxidation are the cause for the observed deactivation. Significant differences in behavior of the Co/Al2O3 and the CoRe/Al2O3 catalyst when exposed to H2O/H-2/He were found by gravimetry, TPR, pulse adsorption and XPS. The CoRe/Al2O3 catalyst was reoxidized more easily in H2O/H-2/He mixtures than the Co/Al2O3 catalyst. This is probably a result of the higher dispersion of the CoRe/Al2O3 catalyst, but a direct influence of Re on the reoxidation cannot be excluded. A phase interacting strongly with the alumina support was found in both catalysts after H2O/He exposure, but also another oxide phase was formed. This second phase is reduced at lower temperature for the CoRe/Al2O3 catalyst than for the Co/Al2O3 catalyst.