In recent years, the supercritical antisolvent (SAS) precipitation technique has emerged as a promising method for the formation of fine particles. Despite its numerous advantages, this technique still cannot be used to produce particles in the, sub-micron range ( < 300 nm) for many, "soft" materials. A significantly improved SAS process can produce particles of controllable size, up to an order of magnitude smaller than those of the conventional SAS process, with a narrower size distribution. Like the conventional SAS technique, this new, supercritical antisolvent with enhanced mass transfer technique utilizes supercritical carbon dioxide as the antisolvent, but the solution jet is deflected by, a surface vibrating at an ultrasonic frequency atomizing the jet into much smaller droplets. Furthermore, the ultrasound field generated by, the vibrating surface enhances mass transfer and prevents agglomeration through increased mixing. The particle size is controlled by varying the vibration intensity, of the deflecting surface, which then can be adjusted by, changing the power supplied to the attached ultrasound transducer. It is demonstrated by the formation of lysozyme nanoparticles and microparticles. The biological activity of the protein is retained during the processing.