Coating or encapsulation of nanoparticles is a major challenge due to the extremely small size, high surface energy, and high surface area of the nanoparticles. In this paper we describe a new method using supercritical CO2 as an anti-solvent (SAS) for nanoparticle coating/encapsulation. A model system, using silica nanoparticles as host particles and Eudragit polymer as the coating material, was chosen for this purpose. The SAS process causes a heterogeneous polymer nucleation with the nanoparticles acting as nuclei and a subsequent growth of polymer on the surface of the nanoparticies induced by mass transfer and phase transition. A polymer matrix structure of encapsulated nanoparticles is formed by agglomeration of the coated nanoparticles. Field emission scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy and Fourier transform infrared spectroscopy were used to characterize the coated/encapsulated silica nanoparticles. (C) 2003 Elsevier B.V. All rights reserved.