The first example of a phase-transfer-catalyzed reaction carriedout in a supercritical fluid (SCF)is reported. The kinetics of the nucleophilic displacement ofbenzyl chloride with a bromide ionin SCF solvent CO2 in the presence of acetone cosolvent ispresented. In addition, the solubilityof tetraheptylammonium bromide (THAB) in SCF CO2 in thepresence of acetone cosolvent isreported. The reaction rate was measured at 50 and 75 °C atpressures to 200 bar; the reactionwas found to follow first-order, reversible kinetics for the reactionscatalyzed by THAB andzero-order kinetics for reactions catalyzed by 18-crown-6.