Carbon isotope transients at reaction steady state are used to examine the effect of water vapor on the amount and reactivity of the surface carbon intermediates involved in the Fischer-Tropsch synthesis on both a supported and an unsupported cobalt catalyst. Water increases the amount of active surface carbon, present predominantly as monomeric species. This increased surface concentration of monomeric carbon is caused by an acceleration of the CO dissociation rate without a matching reactivity increase in the downstream hydrocarbon synthesis steps. In turn, the proposed monomer dependencies in the Fischer-Tropsch synthesis mechanism explain the lower methane selectivity and higher molecular weight products observed at increased water concentrations.