Triplet-triplet annihilation of anthracene in supercritical carbon dioxide with and without acetonitrile cosolvent was studied with a transient absorption technique. In pure CO2, enhancement of the reaction rate was observed. Extinction coefficients and quantum yields of triplet state anthracene in the near-critical region sharply increased and decreased, respectively. In the presence of cosolvent, however, the enhancement of the reaction rate constant in the near-critical region decreased with increasing amounts of cosolvent. Under these conditions, both the extinction coefficients and quantum yields of triplet state anthracene showed less variation providing strong evidence that the cosolvent shields specific interactions between solute and the CO2 solvent. (C) 2002 Elsevier Science B.V. All rights reserved.