The effect of water on the performance of narrow and wide-pore silica-supported cobalt catalysts was investigated during Fischer-Tropsch synthesis in a continuously stirred tank reactor (CSTR). In these studies the added water replaced an equivalent amount of inert gas so that all other reaction conditions remained the same before, during and after water addition. A low cobalt loading of 12.4 wt.% on wide-pore silica exhibited a beneficial effect on CO conversion with the addition of water up to 25 vol.% of the total feed. In contrast, the addition of up to 20 vol.% water to a 20 wt. % Co on narrow- or wide-pore silica did not significantly alter the CO conversion. It appears that the CO conversion mainly increases when cobalt clusters are small enough to fit inside the silica pores. (c) 2005 Elsevier B.V. All rights reserved.