Pure naproxen particles and mixed naproxen-polylactic acid particles were formed by pulsed rapid expansion of supercritical CO2 solutions (RESS). The characterization of the particles was carried out in the aerosol phase as a function of time after particle formation with rapid-scan infrared spectroscopy and with an aerosol particle sizer. After collection of the particles from the aerosol phase, the particles were characterized with scanning electron microscopy and powder diffraction. The present study shows clear evidence that RESS is capable of forming sub-micron naproxen particles that are coated with polylactic acid. Furthermore, it is demonstrated that coating with polylactic acid stabilizes the naproxen particles against agglomeration and coagulation.