Water, the primary oxygen-containing product in Co-catalyzed Fischer-Tropsch synthesis (FTS), increases CO conversion rates and the selectivity to olefins and to C5+ products. These marked rate and selectivity enhancements can reflect changes in the density or reactivity of the active Co surface atoms available during FTS. Kinetic isotope effects and in situ infrared spectroscopy were used to probe possible mechanisms for these water effects. Kinetic isotope effects (r(H)/r(D)) measured from CO/H-2 and CO/D-2 were less than unity for both CO conversion and C5+ formation rates, suggesting that hydrogen adsorption-desorption thermodynamics and C-H bond formation are involved in kinetically relevant FTS steps. The presence of varying amounts of water (H2O or D2O) did not influence the measured kinetic isotope values, even though water in concentration range led to marked changes in FTS rate and selectivity. Thus, the isotopic identity of the water molecules does not influence the rate of kinetically relevant steps, the identity of which appears to remain unchanged by the presence of water. No new pathways are apparently introduced by the presence of water in H-2/CO reactant streams. The intensity and vibrational frequency of adsorbed CO bands were also not influenced by the concentration of water in the reactant stream. These in situ infrared spectroscopic studies showed that water does not influence the density or structure of adsorbed CO intermediates or the number of exposed Co atoms, which bind CO at near-monolayer coverages during Fischer-Tropsch synthesis reactions. These spectroscopic studies also suggest that neither CO transport restrictions nor their removal via the formation of water-rich intrapellet liquids at high water concentrations are responsible for the observed effects of water. These studies are silent about a possible effect of water on the concentration of reactive carbon species, which may be present as minority but kinetically relevant intermediates during FTS.