There is increasing interest in the application of supercritical CO2 (SCCO2) in the preparation of polymer membranes. Membrane formation with SCCO2 as a nonsolvent is analogous to the conventional immersion precipitation process using an organic nonsolvent. Polylactide membranes were prepared with SCCO2 as the nonsolvent under different experimental conditions such as different polymer concentrations, different depressurization rates, and different nonsolvent compositions. The effects of these conditions on the cross-sectional structure were investigated through scanning electron microscopy. In addition, solvent-induced crystallization and CO2-induced crystallization were studied. The crystallinity of PLA membranes prepared with different solvents or at different pressures was characterized by wide-angle X-ray diffraction and differential scanning calorimetry. (c) 2005 Wiley Periodicals, Inc.