The stability of a series of Co/Al2O3 and Re-Co/Al2O3 Fischer-Tropsch (FT) catalysts, with varying Co particle size, was measured in a continuous flow, stirred tank reactor operated at 220 degrees C, 2.1 MPa and with a H-2/CO = 2/1 synthesis gas for periods up to 190 h time-on-stream (TOS). Results showed that catalyst stability was dependent upon the Co particle size, the degree-of-reduction (DOR) of the catalyst precursor, and the CO conversion. At the chosen operating conditions, carbon deposition was the main cause of catalyst deactivation and the initial rate of carbon deposition per active Co site increased with increased Co particle size (d(Co) = 2-22 nm) when measured at approximately the same CO conversion level. On the 15 wt % Co/Al2O3 catalyst the initial rate of carbon deposition increased with CO conversion (CO conversion <= 40%) whereas, on the 1.2 wt %Re-12 wt %Co/Al2O3 catalyst, the initial rate of carbon deposition decreased with increased CO conversion (CO conversions >60%) due to high concentrations of H2O and CO2 in the reactor.