The deactivation rate of Co-Al2O3/SiO2 Fischer-Tropsch synthesis catalyst have been experimentally assessed at industrially relevant conditions (T 198-237 degrees C, P = 60 bar, H-2/CO = 1.85, GHSV 2000-7000 h(-1)). The fresh and spent catalyst was characterized by XRD, XPS, BET, TEM, and SEM-EDX techniques. Decreasing active surface area due to cobalt oxidation by product water and sintering of cobalt crystallites were found to be minor factors for the deactivation. The principal reason of activity loss seems is diffusion limitations arising due to catalyst pores plugging by product wax. The linear dependence between gas flow rate and estimated catalyst lifetime was revealed. High GHSV of fresh synthesis gas and/or high circulation ratio of product gas should be employed to prevent rapid loss of catalyst activity. Graphic