The addition of water during Fischer-Tropsch synthesis over a supported ruthenium catalyst led to a significant increase in product formation rates and significant changes in product selectivity, in particular lower methane selectivity and improved chain growth. Upon increasing water partial pressures, the total product distribution shifted from ASF distributions, with typical deviations due to olefin reinsertion, to a much narrower distributions. Such distributions can mechanistically not be explained by sole C-1-wise chain growth. An additional product formation route considering combination of adjacent alkyl chains to form paraffins ("reverse hydrogenolysis") has been proposed. The findings are discussed with regard to the crucial mechanistic role of water as a moderator in the kinetic regime of the Fischer-Tropsch synthesis.