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This study is designed to examine the effects of supercritical fluid (SCF) solvents (pentane or hexane) on product distributions from Fischer−Tropsch synthesis (FTS) over an alumina-supported cobalt catalyst in a fixed-bed-reactor. The influence of reaction conditions (such as temperature, pressure, syngas feed ratio, contact time, and space velocity) on the FTS hydrocarbon product distributions in the supercritical solvents was studied. Changing the density-dependent bulk thermophysical properties of the supercritical pentane medium, by tuning either temperature (210−250 °C) or pressure (45−65 bar), resulted in notable effects on the (C6+) hydrocarbon product distributions and selectivity. One particular set of experiments was designed to directly compare hexane and pentane solvent effects at constant density (ca., ρ = 0.3 g/cm³) and temperature (240 °C) where significantly higher pressure was required in pentane to achieve this density. Interestingly, both pentane and hexane supercritical FTS performed almost identically at the common density, in terms of hydrocarbon product distribution; however, higher CO conversion was obtained for the pentane phase reaction, because of the pressure effects on the kinetics. Within the investigated range of conditions, the chain-growth probability (α-value) increases as the pressure increases and the temperature decreases; however, the influence of reaction temperature on the chain growth probability was determined to be more pronounced than that of the effect of pressure. Through comparison of hydrocarbon distributions in both SCF-phase and gas-phase FTS, we conclude that the enhanced α-value in SCF−FTS is due to the enhanced solubilities of heavy hydrocarbon products in the SCF medium, thereby improving vacant-site accessibility for the readsorption of olefins and subsequent chain growth, as well as the elimination of the adsorption layer.