A new method-rapid expansion from supercritical solution with a nonsolvent (RESS-N) - is reported for forming polymer microparticles containing proteins such as lysozyme (from chicken egg white) and lipase (from Pseudomonas cepacia). A suspension of protein in CO2 containing a cosolvent and dissolved polymer is sprayed through a nozzle to atmospheric pressure. The polymers are poly(ethylene glycol) (PEG4000; MW = 3,000 PEG6000; MW = 7,500, PEG20000; MW = 20,000) poly(methyl methacrylate) (PMMA; MW = 15,000), poly(L-lactic acid) (PLA; MW = 5,000), poly(DL-lactide-co-glycolide) (PGLA; MW = 5, 000) and PEG - poly(propylene glycol) (PPG) - PEC triblock copolymer (MW = 13,000). The solubilities of these polymers in CO2 increase significantly with low-molecular-weight alcohols as cosolvents. The particles do not tend to agglomerate after expansion, since the pure cosolvent is a nonsolvent for the polymer. The structure and morphology of the microcapsules were investigated by TEM, SEM, and optical microscopy. The thickness of the polymer coating about the protein, as well as the mean particle diameter and particle-size distribution could be controlled by changing the feed composition of the polymer.