The association of formic acid in both carbon dioxide and ethane at 298-318 K and 48-100 bar has been studied using Fourier transform infrared (FTIR) spectroscopy. The equilibrium constant, K, between dimer and monomer of the formic acid was obtained by examination of the carbonyl stretching band for formic acid. The concentrations of formic acid studied ranged from 1.0 to 8.4 mmol/L. In general, the results show that an increase in density causes an increase in the concentration of the formic acid monomer, which results in a decrease in K. The dimerization constant is significantly higher in ethane than in carbon dioxide. This result is a consequence of an enhanced interaction between formic acid (solute) and carbon dioxide (solvent) compared to the interaction between formic acid (solute) and ethane (solvent). Furthermore, the modified lattice-fluid hydrogen-bonding model (MLFHB) has been used to interpret the effects of density on the K. The influence of the carbon dioxide and ethane solvents on the equilibrium behavior of the formic acid is discussed in terms of the role of the solvent density and specific solute-solvent interactions.