Ionic dyes were successfully encapsulated in colloidal polymer particles by a CO2-based microencapsulation technique. The water-soluble dyes were made hydrophobic by forming ion pairs with several types of alkyl quaternary ammonium cations. The hydrophobic ion pairs were then encapsulated in preexisting size monodisperse polystyrene particles dispersed in water. High-pressure carbon dioxide swelled and plasticized polystyrene and thus facilitated mass transport of the dye into the particles. The results show that the particles maintain their size and morphology after exposure to CO2, and that ion-paired dyes have significantly higher loading in the polymer particles than the original dyes. The effects of cosolvent and surfactant are also discussed. (C) 2006 Elsevier B.V. All rights reserved.