Fischer-Tropsch synthesis has been investigated over a commercial type cobalt-based catalyst (20 %Co/0.5 %Re/gamma-Al2O3) by varying the H-2/CO ratio (2.55-1.12), CO conversion (15-75 %), reaction temperature (210 degrees C, 230 degrees C), and by adding water to the syngas. The experiments were conducted in a fixed bed reactor with the main purpose of obtaining experimental data to be used in the development and fitting of a mechanistic model. A positive effect of water on the catalyst activity was found for experiments with a H-2/CO ratio higher than 1.7. Water was found to always increase the C(5+)selectivity regardless of the H-2/CO ratio. Increasing conversion led to increased C(5+)selectivity. The selectivity to CO(2)was significantly enhanced at higher conversions (high water partial pressure), particularly with the lowest H-2/CO = 1.12, interpreted as the emergence of water-gas shift activity. Re-oxidation of cobalt, probably limited to small cobalt particles, is proposed as the main deactivation mechanism caused by water while a steeper deactivation curve was found for higher temperature, indicating that sintering also may play a role.