The rapid expansion of supercritical solutions (RESS) has been applied to separate the hydrocarbon mixture. The RESS experiments for the naphthalene and phenanthrene mixture using supercritical carbon dioxide have been carried out to study the influence of various RESS conditions on separation as well as on morphology. High pre- and postexpansion temperatures enhanced the naphthalene composition of the precipitate more than the preexpansion solution composition. By diluting the preexpansion mixture with pure CO2 and then increasing the postexpansion temperature, an increase in the size of the particles precipitated from mixtures was found. Increasing continuously the phenanthrene composition of the preexpansion mixture solution, the shape of the crystals changed from flat-sheet (pure naphthalene) to needlelike (pure phenanthrene). Most of the experiments were carried out at the extraction temperature of 308.2 K and at the pressure of 14.0 MPa. The highest naphthalene composition of particles precipitated from the saturated fluid mixture (0.883 mole fraction of naphthalene) reached a 0.960 mole fraction of naphthalene at a higher postexpansion temperature than that of the eutectic.