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The effect of water on the activity and selectivity for a series of c-Al 2 O 3 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 c-Al 2 O 3 supports with different pore characteristics. For the narrow-pore catalysts, addition of water corresponding to an inlet partial pressure ratio of P H2 O/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 P H2 O/P H2 =0.7 at the reactor inlet suppressed the reaction rates and led to permanent deactivation. The addition of water increased the C 5+ selectivity and decreased the CH 4 selectivity for all catalysts. The pore characteristics seem to determine the effect of water on the rates.