The first example of a phase-transfer-catalyzed reaction carried out in a supercritical fluid (SCF) is reported. The kinetics of the nucleophilic displacement of benzyl chloride with a bromide ion in SCF solvent CO2 in the presence of acetone cosolvent is presented. In addition, the solubility of tetraheptylammonium bromide (THAB) in SCF CO2 in the presence of acetone cosolvent is reported. The reaction rate was measured at 50 and 75 degrees C at pressures to 200 bar; the reaction was found to follow first-order, reversible kinetics for the reactions catalyzed by THAB and zero-order kinetics for reactions catalyzed by 18-crown-6.