Phase separation phenomena of polyetherimide (PEI)/solvent/nonsolvent systems were investigated by measuring their precipitation values over the temperature range from 20 to 50 degrees C. The solvents used are N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAC), and dimethylformamide (DMF). Nine nonsolvents were employed including water, methanol, ethanol, 1-propanol, 2-propanol, acetic acid, propionic acid, ethylene glycol, and diethylene glycol. Based on the measured precipitation values, critical solubility parameters for PEI were calculated, and the partial solubility boundary for PEI was obtained in a two-dimensional solubility parameter coordinate graph. The relationship between solvent strength and membrane structure was examined using PEI hollow-fiber membranes prepared from binary polymer solutions containing NMP, DMAC, and DMF as solvents. Water was used both as internal and external coagulants. The cross-sectional structure and gas permeation properties of these hollow fibers were examined.