Cellulose acetate (CA) membrane was modified with ultra-thin polyion complex (PIC) layers, and the pervaporation performance for water-ethanol mixture was investigated. Introduction of oxygen-containing anionic groups onto the surface of the CA membrane was attempted by the oxygen plasma treatment, and was confirmed by the electron spectroscopy for chemical analysis (ESCA). The formation of an ultra-thin PIC layer on the membrane surface by dipping of the plasma-treated membrane into an aqueous solution of polyallylamine (PALA) as the polycation was confirmed by the measurement of the attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra of the membrane. The results of the pervaporation of water-ethanol mixture showed that the PIC layer increased the selectivity of the CA membrane. The considerable increase in the selectivity was not accompanied by any large decrease in the water flux. Furthermore, it was observed that the applied energy for plasma treatment strongly affected the permeate flux and the selectivity of the CA/PIC membrane. Further improvement in the selectivity of the membrane was attained by the formation of multilayer PIC using polyacrylic acid (PAA) as the polyanion.