The catalytic hydrogenation of fumaric acid, a prominent bioplatform molecule, was evaluated over PdRe/SiO2 within a variety of reaction media compositions comprising methanol, water, supercritical CO2, and their assorted combinations with H2, across a range of temperature and pressure regimes. Meticulous manipulation of the reaction media composition facilitated the precise modulation of catalytic activity to favor specific hydrogenation products. For instance, within a water-saturated supercritical CO2 medium, γ-butyrolactone exhibited a noteworthy 89% selectivity at 188°C under 250bar total pressure, whereas in the methanol-containing supercritical CO2 system, hydrogenation displayed a remarkable 91% selectivity towards tetrahydrofuran at 175°C and 250bar. This investigation underscores the multifaceted influence of CO2, extending beyond its pressure-derived effects - it functions as a solvent, selectively favoring catalytic pathways and enhancing product selectivity.