When Fischer-Tropsch synthesis (FTS) reaction experiments were conducted in a gas-solid system with a TiO2-supported cobalt catalyst in a continuous stirred tank reactor (CSTR), we observed significant changes in the reaction rate and product selectivity at an early stage of time on stream (TOS) when all the reaction conditions were kept constant.(1) We designed flushing experiments with an inert gas that started when the FTS reaction had reached steady state. After the completion of flushing, the FTS reaction was resumed with syngs. We then compared the results of the FTS reaction rate and product selectivity both before and after flushing, The flushing experimental results suggested that the marked variations we observed were caused (either wholly or mainly) by liquid products deposited in the catalyst rather than by the change in the properties of the catalyst surface. The concentrations and the relative amount of the reactant in the flushed out stream were examined and the implications of the high H-2/CO ratio for the reaction kinetics and product selectivity are discussed. On the basis of the dynamic concentrations of C-1-C-8 in the flushed outgas, we propose that reaction among the products takes place under moderate FTS reaction conditions.