A technique was developed to size-selectively separate polydisperse dispersions of CdSe/ZnS nanocrystals into distinct color fractions using only the tunable solvent properties of CO2-expanded hexane. This size-selective precipitation of semiconductor nanoparticles is achieved by finely tuning the solvent strength of the CO2/hexane medium by simply adjusting the applied CO2 pressure. These subtle changes affect the balance between osmotic repulsive and van der Waals attractive forces, thereby allowing fractionation of the nanocrystals into multiple narrow size populations.