A new process for encapsulation based on the principle of precipitation of nanoparticles by pressure reduction of CO2 gas-expanded liquids (PPRGEL) is presented here. Encapsulation has been studied for two types of core solid nanoparticles, namely suspended and in situ produced particles from solution. Mechanisms for both types of encapsulations have been proposed: for the former type deposition happens because of mass transfer from the bulk solution; for the latter type encapsulation happens if the supersaturation profiles of the core and coating solutes are staggered in time in order that the core solute precipitates first and the coating solute deposits on it by mass transfer. A model for the process that includes nucleation and growth has been developed to select systems and process conditions that favor encapsulation. The mechanisms have been experimentally verified at 52 bar and 303 K for (i) encapsulation of suspended nanoparticles of silica with ascorbylpalmitate (AP) dissolved acetone and (ii) encapsulation of in situ produced tartaric acid (TA) nanoparticles with AP, both initially dissolved in acetone. A uniform coating of about 10-20 nm of AP is formed on the 250 nm silica particles. For the two-solute system it is observed that AP deposits on TA resulting in encapsulated particles of an average size of about 520 nm. (C) 2013 Elsevier B.V. All rights reserved.