The adsorption isotherms and the effective diffusivities of benzene in activated carbon pellets in the presence of subcritical and supercritical carbon dioxide were measured from the sorption curves in a spinning basket autoclave at temperatures 308, 318, and 328 K and pressures up to 163 bar. The adsorption isotherm data showed that no condensation of benzene in activated carbon occurred at the given temperatures and pressures. The effective diffusivities measured were observed to be concentration-dependent and to decrease with increasing pressure. At high-pressure operations, the effective diffusivities were found to be invariant with temperature.