Quercetin is a strong naturally occurring antioxidant that is exploited in pharmaceutical and cosmeticsapplications. Unfortunately, quercetin is highly susceptible to oxidation. Besides, its poor solubility inwater and low bioavailability upon oral administration limit the use in drug formulations for the treat-ment of human diseases. In an effort to overcome these drawbacks, the micronization and coprecipitationof quercetin particles with a low-cost biocompatible polymer (ethyl cellulose, EC) was studied by usingsupercritical anti-solvent process (SAS) with a non-toxic solvent ethyl acetate. The results showed thatSAS micronization of quercetin led to a reduction in the quercetin particle size and crystallinity withouta change in the needle-like habit. SAS coprecipitation of quercetin with EC at moderate pressure andtemperature (10 MPa and 35◦C) led to obtaining quasi-spherical particles. The coated polymer avoidthe growth of quercetin crystals, thus amorphous particles in the submicron range (mean size rangingbetween 150 and 350 nm) were formed. Promising coprecipitation results were reached with quite highprocess yields (above 85%) and encapsulation efficiencies up to 99% that provided a high stability to thecoated quercetin with EC against oxidation.