Fischer-Tropsch Synthesis (FTS) is the catalytic conversion of syngas into several products such as hydrocarbons, oxygenates and H2O. H2O is a key by-product of the FTS process and because a Co based catalyst has a low water-gas shift activity, the concentration of H2O increases with time on stream in FTS. While H2O has a positive influence on C5+ selectivity and a negative impact on CH4 selectivity, for virtually every Co based catalyst, the same cannot be said for the activity of all Co based catalysts, because H2O affects the activity of various Co based catalysts in different ways. Most authors observe a positive H2O effect for SiO2-supported Co catalysts and TiO(2-)supported Co catalysts; however, with gamma-Al2O3-supported Co catalysts, a negative influence of H2O is widely reported. In addition, the impact of H2O on carbon nanofiber-supported Co catalysts is reported in literature to be positive. We propose that the phase of H2O in FTS and the water-affinity and/or resistance behavior of catalyst supports could influence the activity and selectivity of Co-based catalysts and that this could help in explaining the kinetic effect of H2O in FT synthesis.