The distribution coefficients of naphthalene between slitpores and supercritical carbon dioxide at infinite dilution are calculated by the Monte Carlo simulation method in the range of entrainer (benzene) concentration from 0 to 2%. The addition of a small amount of benzene decreases the distribution coefficient dramatically. It is found that added benzene molecules prefer to adsorb onto the wall surface in the pore phase, which increases the pore density and excludes naphthalene molecules.