Fourier transform infrared (FT-IR) spectroscopy has been used to measure the molarities of hydrogen bonding species between carboxylic acids (acetic acid and palmitic acid) and water in supercritical CO2. The equilibrium constants of dimerization for the carboxylic acids were determined in supercritical CO2 with octane. Further, the interactions of propanol-d (1- and 2-propanol-d) or xylenol (2,5-, 2,6- and 3,4-xylenol) isomers with acetone in supercritical CO2 were studied. Experiments were carried out at 308.2-313.2 K and 7.0-20.0 MPa. The molarities of hydrogen bonding species between the carboxylic acids and water in supercritical CO2 increase with the increasing molarity of water. The carboxylic acids interact more easily with ethanol than water in supercritical CO2. For supercritical CO2 + carboxylic acid + octane systems, the equilibrium constants between the carboxylic acid monomer and dimer increase with the increasing molarity of octane. The equilibrium constants of the carboxylic acids seem to approach to those in liquid paraffin according to addition of octane in supercritical CO2. The amount of the interaction species between 1-propanol-d and acetone is larger than that between 2-propanol-d and acetone. The amount of acetone interacting with OH group for 3,4-xylenol is the largest among those for xylenol isomers. These differences among the isomers may be caused by the screen effects of methyl groups around hydroxyl group for the isomers.