Sorption and permeation characteristics of pure water and ethanol vapors, and permeation behavior of their mixed vapor in poly(vinylchloride) (PVC) membranes were investigated at 40 degrees C. Equilibrium sorption isotherms of pure water and ethanol vapors in the PVC membrane were well described by the dual-mode sorption model, All the dual-mode sorption parameters of ethanol vapor were higher than those of water vapor. Permeabilities for pure water and ethanol vapors of the PVC membrane decreased with increasing vapor pressure. This behavior was analyzed by the dual-transport model. It was found that the diffusion coefficient of water Vapor in the PVC membrane was much higher than that of ethanol vapor. In the binary vapor mixture of water and ethanol, the permeability of water vapor was high and that of ethanol vapor was lower, comparing to the expectation from the dual-transport model. The actual separation factor for water was much higher than the value presumed from the model.