The effect of water on the catalytic properties of a Co/SiO2 catalyst (12.4 wt.%) during Fischer-Tropsch (FT) synthesis was investigated in a continuously stirred tank reactor (CSTR) using two approaches: (1) changing the space velocity to change the CO conversion and therefore the water partial pressure and (2) by adding varying amounts of water into the feed gas. Even at a space velocity of 1 SL/g catalyst/h at a CO conversion of about 75%, the catalyst displayed very high stability. The addition of water in the range of 5-25% (vol.%) increased the CO conversion. Continuous addition of small amounts of water (5 vol.%) over a period of days caused an increase of CO conversion and did not have a significant effect on the catalyst deactivation; however, for larger amounts of water, continuous addition resulted in a severe catalyst deactivation.