Pervaporation of ethanol-water mixture was examined on IPN membranes composed of hydrophilic polyurethane(PU) and hydrophobic polystyrene(PS). The IPN membranes showed preferential pervaporation of water over ethanol and revealed high permeation rate. As the content of hydrophobic PS was increased. The permeation rate decreased while the separation factor increased, indicating that the PS domains suppressed the swelling of the PU phase and reduced the plasticizing effect. The average diffusion coefficient, computed from the permeation rate and solubility, was highly dependent on the viscosity and concentration of the permeant in the membranes.