The effect of water partial pressure on the Fischer-Tropsch (FT) rate of a cobalt catalyst supported on narrow-pore gamma-Al2O3 was investigated at industrially relevant process conditions (483 K, 30 bar, pellet size: 53-90 mu m). Inlet water partial pressure was varied up to 9 bar by external water vapour addition at different H-2/CO molar ratios ranging from 1 to 3. The effect of water was found to be positive on FT-rate independently of the H-2/CO ratio, but more significantly at H-2-poor condition. Temperature-programmed hydrogenation (TPH) was used to verify the presence of unreactive carbon species on the catalyst after 22 h on stream at the different conditions with and without exposure to about 9 bar water. A higher temperature feature that could be associated to amorphous polymeric carbon was detected at H-2-poor conditions but remained unchanged upon 2 h of water exposure which did not result in a change in the amount of amorphous polymeric carbon detectable by TPH. (C) 2012 Elsevier B.V. All rights reserved.