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The extent of diffusion limitations in the catalytic conversion of synthesis gas to hydrocarbons by the Fischer-Tropsch reaction has been established for a number of iron-and cobalt-based catalysts. The studies were performed in a fixed-bed microreactor system at temperatures in the range 473-523 K. Variation of catalyst particle size in the range 0.2-2.6 mm shows that the conversion of synthesis gas decreases considerably when the average particle size is increased. The effects of variation of particle size and pore diameter have been quantified with the Thiele model for diffusion limitations. Evidence has accumulated that the limited mobility of reactant molecules in the liquid-filled pores of Fischer-Tropsch catalysts is the main cause of retardation of the reaction rates. The experimentally determined reaction rates with various catalysts operated under different conditions show an excellent fit with the theoretical model.