Membrane formation using a compressed antisolvent is analogous to conventional immersion precipitation using liquid nonsolvents and introduces pressure as an additional variable for tailoring the membrane microstructure. Thin films of a semicrystalline polyamide, Nylon 6, were precipitated from 2,2,2-trifluoroethanol by exposing the incipient membrane to compressed CO2 antisolvent at 35 degreesC and variable pressures up to 173.4 bar. Membrane structures dominated by liquid-liquid (L-L) and solid-liquid (S-L) demixing processes were observed as a function of precipitation conditions. Interpretation of the resulting membrane morphologies was based on structural features observed for traditional phase inversion processes and reflected the relative rates of L-L and S-L demixing as a function of the pressure-dependent strengths of the solvent and antisolvent. The ability to tailor the morphology of a semicrystalline membrane using compressed antisolvent suggests an alternative to current CO2-based polymer impregnation techniques, which require solute solubility in CO2, for the generation of composite thin films and membranes.