The assocn. of formic acid in both carbon dioxide and ethane at 298-318 K and 48-100 bar has been studied using Fourier transform IR (FTIR) spectroscopy. The equil. const., K, between dimer and monomer of the formic acid was obtained by examn. of the carbonyl stretching band for formic acid. The concns. of formic acid studied ranged from 1.0 to 8.4 mmol/L. In general, the results show that an increase in d. causes an increase in the concn. of the formic acid monomer, which results in a decrease in K. The dimerization const. 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 d. on the K. The influence of the carbon dioxide and ethane solvents on the equil. behavior of the formic acid is discussed in terms of the role of the solvent d. and specific solute-solvent interactions.