Polymer microparticles are useful for numerous applications such as stationary phases in chromatography, adsorbents and catalyst supports, as well as for drug delivery systems. In recent decades the application of supercritical fluids for microparticle precipitation has been developed to a point where it is an ideal alternative to conventional processes. In this work poly(L-lactic acid) (PLLA), a biodegradable and biocompatible thermoplastic aliphatic polyester, has been processed using supercritical fluids, particularly by rapid expansion of supercritical solutions (RESS) and supercritical antisolvent (SAS) processes over a wide miscibility range. Particle morphology was greatly improved from irregular blocks to spherical microparticles on applying the SAS process. The effects of changes in polymer concentration, liquid flow rate, nozzle diameter, solvent, pressure and temperature have also been evaluated on the particle size of PLLA in the SAS precipitation. A higher concentration of the initial solution led to a decrease in particle size. Dichloromethane was the best of the chlorinated solvents investigated. The nozzle diameter had a negligible effect on particle size and the highest liquid flow rate gave the largest particle size. A larger particle size was also obtained on increasing the operating temperature. In contrast, the particle size decreased on increasing the operating pressure. (C) 2013 Elsevier B.V. All rights reserved.