The thermal decomposition of 2,2'-azobis(isobutyronitrile) (AIBN) in supercritical CO2 was studied at 335.15 K, and at 12 and 14 MPa by using UV/Vis spectroscopic method. The effect of co-solvents methanol and cyclohexane on the decomposition rate was also investigated. The co-solvents, especially methanol, can accelerate the decomposition considerably, which is attributable to local composition enhancement. The local concentration of the co-solvents around the reaction species was estimated, and the results showed that the local concentration of the cosolvent can be 20 times of that in the bulk, which depends on pressure and co-solvent concentration. (C) 2001 Elsevier Science BN. All rights reserved.