In this study, peptide-loaded microparticles were prepared using an aerosol solvent extraction system (ASES) by employing supercritical carbon dioxide as an antisolvent. The effects of the molecular weight of poly(L-lactide) (PLLA), poly(ethylene glycol) (PEG), the block length of methoxy poly(ethylene glycol)-b-poly(L-lactide) (mPEG-PLLA), the blending of PLLA and PEG, and the drug-to-polymer feed ratio on the formation of leuprolide acetate (LA)-loaded microparticles and their release characteristics were investigated. Scanning electron microscope observations showed that the LA-loaded polymer particles had a spherical morphology with a smooth surface. The entrapment efficiency of LA in the ASES-processed microparticles was found to be extremely high (about 99%), whereas the initial release rate of the LA-loaded microparticles was very low for PLLA. The release rate of LA was observed to increase as the PEG block length of mPEG-PLLA and/or the drug content in the microparticles increased. When PLLA was blended with PEG, the release rate of LA from the PLLA/PEG microparticles was significantly faster compared with the corresponding mPEG-PLLA copolymer.