This paper demonstrates the rapid and precise size selection of nanoparticle populations using the pressure tunable solvent properties of CO₂-expanded liquids. Specifically, by pressurizing and expanding a single organic solution with carbon dioxide gas, ligand-stabilized silver particles of desired mean size were size selectively precipitated at desired locations. Compared to current techniques, this CO₂-expanded liquid approach provides for faster and more efficient particle size separation, reduction in organic solvent usage, and pressure tunable size selection.