The extraction of pentachlorophenol (PCP) from pressure-treated wood wafers with supercritical carbon dioxide (SC-CO2) was studied in a continuous-flow extractor. PCP extraction rates were determined as a function of pressure (17.5-25 MPa), temperature (313-353 K), flow rate (1-3 mL/min at supercritical conditions), and sample size(0.8 x 10 x 50 mm and 2.2 x 10 x 50 mm) by measuring PCP concentrations in the extractor effluent intermittently. The rate of extraction increased with an increase in solvent pressure and a decrease in particle size. A fundamental model was developed which includes rates of intraparticle diffusion, external film mass transfer, linear desorption isotherms, and initial distribution of PCP between pore volume (cell lumen) and pore surfaces (cell wall) of wood wafers. The overall mass transfer coefficient and the rate of extraction increased with an increase in solvent pressure, temperature, and flow rate. The adsorption equilibrium coefficients of PCP with wood substance were very small, and more than 80% of the PCP was found to be in the cell lumen initially.