Mechanisms of phenanthrene desorption from five subsurface materials in supercritical carbon dioxide (SC CO2) were investigated by measuring isotherms in the presence of fixed quantities of a polar cosolvent (methanol) and by calculating desorption enthalpies from the temperature dependence of the isotherms. The addition of 7.4 mol % methanol to the SC CO2 phase resulted in 2−11-fold reductions in Freundlich capacity factors at 120 atm and 50 °C. The capacity reduction greatly exceeded that expected from the 21% increase in phenanthrene solubility accompanying cosolvent addition. Isotherms became more linear at 120 atm and 50 °C upon methanol addition for all sorbents except a shale sample. Solubility and organic carbon-normalized phenanthrene sorption capacities declined with increasing solvent polarity, in the order dry SC CO2 > methanol-amended SC CO2 > aqueous solution, and declined with sorbent organic carbon content. Sorption enthalpies from an ideal gas reference state ranged from −106 to −70 kJ/mo...