Coating and encapsulation of fine particles with polymer using a supercritical antisolvent (SAS) coating process was investigated in this research. Synthesized submicron silica particles were used as host particles and poly(lactide-co-glycolide) (PLGA), a biodegradable polymer used for controlled release of drugs, was chosen as the coating material. In the SAS coating process a suspension of silica particles in an acetone-polymer solution was sprayed through a capillary nozzle into supercritical (SC) CO2 which acts as an antisolvent for the acetone. A rapid mutual diffusion between the SC CO2 and the acetone causes supersaturation of the polymer solution, leading to nucleation and precipitation of the polymer to encapsulate the silica particles. The operating parameters that have an effect on the coating process, such as polymer to particle weight ratio, polymer concentration, temperature, pressure, flow rate of polymer solution, and the addition of a SC CO2 soluble surfactant, were systematically studied. It is shown that the polymer to silica ratio and the polymer concentration are critical for the successful encapsulation of silica particles with minimum agglomeration. (C) 2005 American Institute of Chemical Engineers.