Operating Fischer−Tropsch synthesis (FTS) in supercritical fluid (SCF) media offers advantages over conventional gas-phase FTS operation including in situ extraction of heavy hydrocarbons from the catalyst pores coupled with enhanced incorporation of α-olefins in the chain growth process. In this study, hydrocarbon product distributions in near-critical and supercritical hexane phase FTS (SCH-FTS) was studied over a 15% Co/Al₂O₃ in a high-pressure fixed-bed reactor system. The critical point of the hexane−syngas−products reaction mixture as collected from the reactor outlet was measured using a variable-volume view cell apparatus. All reactions were carried out in near-critical and supercritical regimes by tuning either the reaction temperature (230−260 °C) or the reaction pressure (30−80 bar). Deviations from the standard Anderson−Schultz−Flory (ASF) chain growth model were observed in most cases; however, the degree of deviation depends on the reaction conditions within the near-critical and supercritical regions and varies from gaslike density to liquidlike density within the supercritical region. As an attempt to understand this phenomenon, a modification to the chain growth model in SCH-FTS, is presented. The model attributes the deviation from the standard ASF model to enhanced α-olefin incorporation within the middle distillate hydrocarbon products due to enhanced adsorption/desorption dynamic in the SCF medium. The proposed enhanced olefin incorporation model was found to be in a good agreement with our experimental results.