The well-known phenomenon of partitioning of bioparticles in two-phase aqueous polymeric systems is extended to a variety of polymer (acrylic latex) and inorganic (TiO2) colloidal particles. It is demonstrated that particle partitioning into a particular phase is controlled in rational ways by particle surface chemistry, vie., hydrophobicity, functional groups such as hydroxyl and carboxyl, and pH effects, in relation to the chemical nature of the water soluble polymers (WSPs) that comprise the phase-separated system. WSPs include poly(ethylene oxide), poly(N-vinylpyrrolidone), dextran, and a hydrophobe-terminated water soluble polyurethane based on polyethylene oxide. Mixtures of colloidal particles can be separated by selective partitioning.