The absolute reactivity of triplet benzophenone ((BP)-B-3) and benzyl free radical (PhCH(2)) toward molecular oxygen (O-2) in supercritical CO2 and CHF3 has been measured by laser flash photolysis (LFP). The transient reactants may be considered to be infinitely dilute solutes reacting with a gaseous cosolvent in a supercritical fluid mixture. Both reactants were found to undergo kinetically controlled reactivity with O-2 and the measured bimolecular rate constants (k(bi)) were found to decrease with a decrease in solvent density at reduced pressures between 1.0 and 2.5. These results are consistent with solute reactivity with a "nonattractive" cosolvent. The results are compared with those previously obtained for the reaction of (BP)-B-3 with an "attractive" cosolvent, 1,4-cyclohexadiene, in supercritical CO2 and CHF3, in which enhanced (BP)-B-3 reactivity was observed due to preferential cosolvent/solute solvation. Integral equation theory has also been applied to model these ternary systems, and the results indicate how the strengths of local solvation forces can influence kinetically controlled reactions in supercritical fluids. These results represent the first corroborative analysis using absolute reaction kinetics and integral equation theory to probe preferential cosolvent solvation in ternary supercritical fluid mixtures.