FTIR spectroscopy was used to study the hydrogen bonding of acetic acid and tetrahydrofuran in supercritical (SC) CO2 in the ranges of 100-160 bar and 308.15-323.15 K, and the concentration of tetrahydrofuran varied from 0 to 0.4 mol/L. The equilibrium constant K-c for the hydrogen bonding of tetrahydrofuran with the monomer of acetic acid was calculated on the basis of the IR spectra determined. K-c is a strong function of pressure or the apparent density of CO2 (moles CO2 of per liter). The enthalpy of the hydrogen bonding was calculated on the basis of the relationship between K-c and temperature. In the lower isothermal compressible region of the fluid, both the enthalpy change (Delta H) and the entropy change (Delta S) of the hydrogen bending process are negative and the hydrogen bonding is enthalpy driven. In the high isothermal compressible region, however, both Delta H and Delta S are positive, and the hydrogen bonding process becomes entropy driven.