For Fe-catalyzed FischerTropsch (FT) synthesis with near-critical n-hexane (Pc, = 29.7 bar; Tc, = 233.7°C) as the reaction medium, isothermal pressure tuning from 1.2–2.4 Pc, (for n-hexane) at the reaction temperature (240°C) significantly changes syngas conversion and product selectivity. For fixed feed rates of syngas (H2/C0 = 0.5; 50 std. cm3/g catalyst) and n-hexane (1 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 α ( =0.78) Anderson-Schulz-Flory product distribution, the constant chain termination probability, and the higher primary product (1 -olefin) selectivities ( ∼ 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.