Poly(ether imide) segmented copolymers were prepared from polyether diamine and comonomer diamine with acid anhydride. They had microphase-separated structure consisting of microdomains of rubbery polyether segments and of glassy polyimide segments. Pervaporation (PV) of benzene-cyclohexane, benzene-n-hexane and acetone-cyclohexane mixtures through membranes of the copolymers were investigated. As to polyether segment, poly(ethylene oxide) (PEG) gave much better membrane performance than poly(propylene oxide) and poly(tetrahydrofuran). With an increase in PEO content, [PE], the specific permeation flux, Ql, increased significantly and the separation factor, alpha, decreased. Sorption and permeation occurred in microdomains of polyether segments, whereas microdomains of polyimide segments contributed to suppression of swelling of polyether microdomains and to film-forming ability. The membranes were preferentially permeable to benzene or acetone over cyclohexane or n-hexane due to preferential sorption and diffusion of benzene or acetone. The block length of the PEO segment, n, affected the membrane performance; lower Ql and higher alpha were obtained for shorter block length. A typical membrane with [PE]=41 wt% and n=9 displayed Ql=2 kg mu m/(m(2)h) and alpha=9 at a feed composition of 60 wt% benzene in cyclohexane and Ql=8 kg mu m/(m(2)h) and alpha=17 at a feed composition of 68 wt% acetone in cyclohexane at 323 K. Argon plasma treatment was effective for enhancement of the selectivity without significant reduction in el, especially for the acetone-cyclohexane system.