Homogeneous dense membranes of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) were used to separate mixtures of ethanol-organic liquid by pervaporation. If the organic liquid had a strong affinity to PPO (e.g., toluene) and its concentration in the feed was low, smaller molecules of ethanol permeate preferentially through the membrane. When the concentration of this liquid in the feed was increased up to a certain value, the membrane was, on the contrary, permeated by it preferentially. When its concentration in the feed was further increased, flux through the membrane is dramatically increased and the membrane lost its selectivity. If the liquid in a binary mixture with ethanol in the feed had a low affinity to PPO (e.g., 1-propanol), the selectivity of the PPO membrane virtually did not depend on the feed composition, and the membrane was preferentially permeated by smaller molecules.