The solubility of naphthalene in supercritical carbon dioxide fluids has been evaluated by means of the integral equation method. The role of entrainer was investigated by examining the pressure dependence of solubility curves in fluids with or without entrainer. The solubility of naphthalene increases with the addition of a small amount of co-solvent. This increase in solubility arises from the attractive interaction between the co-solvent and the solute naphthalene and from a decrease in pressure at the same density due to stronger interactions between carbon dioxide and the co-solvent than that for carbon dioxide dimer.