Monte Carlo simulations are used to quantify hydrogen bonding between formic acid molecules solvated in subcritical and supercritical carbon dioxide. We find that formic acid predominantly forms cyclic dimers at the conditions studied, in contrast to the chainlike configurations observed in the neat liquid phase. Previous experimental measurements of the dimerization constant are confirmed in the present work, showing large shifts in hydrogen bonding as the two-phase system become supercritical. At subcritical conditions, formic acid is preferentially solvated in the liquid CO2 phase, where the dimerization constant is low. As the system approaches the critical point, the formic acid molecules migrate toward the gas phase, where the dimerization is enhanced. Above the critical point of the mixture, formic acid is completely solvated in a single phase, allowing pressure-tunable dimerization. The results from these simulations prompt future investigations of more complex reactions solvated in supercritical solutions as equilibrium reactions may be efficiently optimized by performing similar computational studies.