The precipitation polymerization of acrylic acid (AA) in supercritical (SC) carbon dioxide with acetic acid and ethanol as the cosolvent were studied at 335 K in the range 12-17 MPa; 2,2'-azobis(isobutyronitrile) AIBN was used as the initiator. Analysis by viscosity measurement and differential scanning calorimetry (DSC) indicates that the cosolvents have a pronounced effect on the product molecular weight and glass transition temperature (T-g) Scanning electron microscopy (SEM) shows that the products are microparticles, and their sizes are affected by the structure and concentration of the cosolvents. A UV/Vis spectroscopic method was used to study the initiation rate of AIBN. The molecular weights of the products are closely related to the decomposition rate.