A new microencapsulation technique is presented in which carbon dioxide is used to facilitate the transport of additives into colloidal polymer particles. Aqueous latexes were impregnated with additives by emulsifying carbon dioxide and water with the aid of a surfactant, Pluronic F108. Ethanol-based latexes were also successfully impregnated by dispersing additives into an ethanol-carbon dioxide solution. Preexisting size monodisperse polystyrene microspheres were impregnated with dyes with varying solubility characteristics. The effects of particle size, surfactant concentration, dye amount, impregnation time, and pressure on the dye loadings were studied. Partitioning of the dye between the polymer phase and the medium surrounding the polymer particles is the driving force for the dye to enter the polymer phase. Consequently, the partition coefficient is the most important factor that determines the maximum dye loading at the thermodynamic equilibrium. Increasing the solubility of the dye in the medium improves the kinetics but results in low dye loading. Dye loading is improved by increasing the interfacial area, either by increasing surfactant concentration or by lowering particle size. Data reported in this paper illustrate high potential of this technique for microencapsulation at room temperature without organic solvents and with precise control of particle size.