Integral asymmetric membranes for gas separation were prepared by casting a poly(ether imide) solution in tetrahydrofuran (THF) and gamma-butyrolactone (GBL) and coagulating in water. As observed by electron microscopy, very thin selective layers ( < 0.5 mu m) were obtained with high GEL contents or when butanol was also added to the casting solution, resulting therefore in the highest permeability values (up to 4671 He/m(2) h bar for membranes prepared from casting solutions with 56 wt% GEL). He/N-2 selectivity values as a function of solvent composition went through a maximum. Maximum values as high as 131 were obtained with equal amounts of THF and GEL in the solvent mixture. The kinetics and mechanism of membrane formation were investigated by light scattering.