The design of high performance pervaporation membranes for the selective removal of ehtanol from ethyl t-butyl ether (ETBE) and toluene from heptane was performed by using semi-interpenetrating polymer network (s-IPN) materials. With cellulose acetate butyrate as the linear polymer and the polyethylene glycol dimethacrylate (PEGDMA) as the monomer to be photopolymerized to form the s-IPN materials, membranes with good mechanical and separation properties were obtained for both applications. The nature of the gas (nitrogen or air) above the polymer solution during the photopolymerization step changes the s-IPN structure in the membrane thickness, making it possible to prepare both homogeneous and chemically asymmetric membranes from the same dope formulation. The solvent diffusion characteristics depend on several parameters; the most interesting one is the nature of the membrane face in contact with the liquid medium in the pervaporation through a chemically asymmetric membrane. When the appropriate face of asymmetric membranes is used in contact with the liquid medium, the ratio of the two solvent diffusion coefficients becomes much higher, and an improvement in the pervaporation selectivity from that of the homogeneous membranes can be excepted.