The solubility of ketoprofen and aspirin in subcritical and supercritical CO2 was measured using FTIR absorption spectroscopy in the large range of temperature of 298.2-353.2 K and pressure of 5-35 MPa. The evolution of the solubility of both active pharmaceutical ingredients (APIs) was fitted using the Chrastil's equation. In addition, the speciation of both APIs in monomeric and dimeric forms was explored by analyzing the characteristic carbonyl stretching vibrations of the carboxylic acid functions assigned to the dimers and monomers, respectively. Moreover, the evolution of the dimerization constant K of the two drugs as a function of the temperature and the pressure of scCO(2) has been reported.