The phase equilibrium of acrylic acid (AA)/carbon dioxide (CO2) as well as mixtures containing water, AA, CO2 and/or ethanol (EtOH) has been investigated. An in-situ infrared analysis method combining FTIR and ATR-IR spectrometry techniques has been used to obtain concentrations of each component in both phases, i.e. CO2-rich and water-rich ones, as a function of pressure and temperature. Measurements have been performed at 65 and 75 °C for pressures ranging between 0.1 and 20 MPa. The solubilities of AA, water and EtOH in the CO2-rich phase and the concentrations of AA, EtOH and CO2 in the water-rich phase have been obtained. From these measurements, a complete phase diagram of the binary AA-CO2 system as well as the partition coefficient of each component in the quaternary system have been established. It has been demonstrated that AA is in equilibrium between the water- and the CO2-rich phases and that EtOH limits this transfer. Nevertheless, aqueous droplets of AA in CO2 at high pressure can be used as monomer reservoirs releasing the monomer in the CO2-rich phase.