The kinetics of Fischer-Tropsch synthesis (FTS) for 12 and 25% Co/Al2O3 and Co/SiO2 catalysts were studied in a 1-L continuously stirred tank reactor (CSTR) in order to define the impact of water on the FTS rate as a function of cobalt crystallite size, as measured by H2 chemisorption/pulse reoxidation experiments. An empirical kinetic model, rCO = k PCOaPH2b/ (1 + m PH2O/PH2), was used in this study. Intrinsic kinetic parameters were identified under a wide range of experimental conditions of 3.3 - 22 Nl-gcat/h, H2/CO ratio of 1.0-2.5, 205-220 oC and a fixed CO pressure of 0.51 MPa and a fixed hydrogen pressure of 0.81 MPa. CO conversion during entire kinetic testing is below 48%. Interestingly, the support type governed whether the impact of H2O on the rate was positive or negative, with less interacting Co/silica providing a positive kinetic effect, while the strongly interacting Co/alumina resulted in a negative impact on the CO conversion rate.