The influences of the water partial pressure and the syngas pressure on the reaction kinetics of a commercial alumina-supported cobalt Fischer-Tropsch (FT) catalyst were investigated. Both a fresh catalyst, as well as an aged catalyst from a demonstration reactor, were employed in the study. It was concluded that water has a negligible influence on the overall FT reaction rate but lowers the methane selectivity and increases the CO2 selectivity. Longer term exposure to higher water partial pressures did not lead to step changes in the catalyst activity but may have increased the rate of gradual irreversible catalyst deactivation. Increasing the syngas pressure at constant H-2/CO ratio significantly increased the FT reaction rate and decreased the methane selectivity, suggesting a substantial incentive to explore higher pressure operation of commercial cobalt-FT processes.