Blend membranes were prepared by casting chloroform solutions of mixtures of styrene-N-phenylmaleimide copolymer containing ca. 10 mol % imide units with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), and their phase behavior was evaluated. At a content of 20% PPO, the membranes were homogeneous; those having 40 or more % PPO exhibited phase separation. Membranes made of the parent polymers and their blends were tested in the pervaporation of aqueous ethanol solutions; permeabilities to oxygen, nitrogen, and carbon dioxide were also determined. The pervaporation characteristics and gas transport properties are discussed considering the interactions of polymer chains and the phase structure of the membranes.