Highly contaminated (with PAHs) topsoils were extracted with supercritical CO2 to determine the feasibility and mechanism of supercritical fluid extraction (SFE). Effects of supercritical fluid (SCF) density, temperature, and cosolvent type and amount and of slurrying the soil with water were investigated. Experiments were conducted at 308 - 343 K and 450 - 850 kg/m(3) CO2 density. Water and methanol were used as cosolvents in some experiments, at 0.3 - 6 wt %. For extractions at an similar to 1000 phase ratio (wt of CO2/wt of soil), near-complete contaminant removal was achieved. For an similar to 200 phase ratio at 323 It and 13.8 MPa, > 95 % of two-ring PAHs and > 80 % of three-ring and heavier PAHs were extracted. Using soil slurries with water had little effect on the total recoveries. Changes in SCF density also had little influence on SFE, while temperature increases improved both initial rates of extraction and total recoveries. Addition of cosolvents generally hindered extraction of the contaminants. These results are used to show that, at typical conditions for CO2-based soil extractions, the solubility of the contaminant in the SCF does not limit SFE. Other internal resistances are important, such as desorption from and diffusion within the polymeric soil organic matter.