The effect of water on the activity and selectivity for a series of gamma-Al2O3 supported cobalt Fischer-Tropsch catalysts has been studied in an isothermal fixed-bed reactor at T = 483 K, P = 20 bar, and H-2/CO = 2.1. The catalysts were produced applying incipient wetness impregnation and consisted of 20 wt.% cobalt and 0.5 wt.% rhenium deposited on gamma-Al2O3 supports with different pore characteristics. For the narrow-pore catalysts, addition of water corresponding to an inlet partial pressure ratio of PH2O/P-H2 = 0.4 reduced the reaction rates. In contrast, for a catalyst with larger pores the same water pressure increased the reaction rates. For all catalysts, water amounts equal to PH2O/P-H2 = 0.7 at the reactor inlet suppressed the reaction rates and led to permanent deactivation. The addition of water increased the C5+ selectivity and decreased the CH4 selectivity for all catalysts. The pore characteristics seem to determine the effect of water on the rates.