This work reports the first studies designed to examine the detailed mechanism of a phase-transfer catalyzed reaction between a supercritical CO2 phase and a solid salt phase. The nucleophilic displacement of benzyl chloride with potassium cyanide to form phenylacetonitrile and potassium chloride was carried out with a tetraheptylammonium salt as the phase-transfer catalyst, The effects of various reaction variables on the kinetics were investigated, including the amount of catalyst, the amount of potassium cyanide, the presence of acetone cosolvent, and temperature. The kinetic data, along with catalyst solubility measurements, indicate that. the operating reaction mechanism is a three-phase system consisting of a supercritical CO2 phase, a catalyst-rich phase, and a solid salt phase and that the reaction actually occurs in the catalyst-rich phase. Further, the reaction mechanism was investigated with two additional phase-transfer catalysts, 18-crown-6 and poly(ethylene glycol), and these results are consistent with the postulated three-phase mechanism.