For Fe-catalyzed Fischer-Tropsch (FT) synthesis with near-critical n-hexane (P-c= 29.7 bar; T-c=233.7 degrees C) as the reaction medium, isothermal pressure tuning from 1.2-24 P-c (for n-hexane) at the reaction temperature (240 degrees C) significantly changes syngas conversion and product selectivity For fixed feed rates of syngas (H-2/CO = 0.5; 50 std. cm(3)/g catalyst) and n-hexane (I mL/min), syngas conversion attains a steady state at all pressures, increasing roughly threefold in this pressure range. Effective rate constants, estimated assuming a first-order dependence of syngas conversion on hydrogen, reveal that the catalyst effectiveness increases with pressure implying the alleviation of pore-diffusion limitations. Pore accessibilities increase at higher pressures because the extraction of heavier hydrocarbons from the catalyst pores is enhanced by the liquid-like densities, yet better-than-liquid transport properties, of n-hexane. This explanation is consistent with the single alpha ( = 0.78) Anderson-Schulz-Flory product distribution the constant chain termination probability, and the higher primaly product (1-olefin) selectivities ( similar to 80%) observed at the higher pressures. Our results indicate that the pressure tunability of the density and transport properties of near-critical reaction media offers a powerful tool to optimize catalyst activity and product selectivity during FT reactions on supported catalysts.