In this work, the pervaporation performance and mechanism of water-ethanol mixtures through symmetric and asymmetric TPX [poly(4-methyl-1-pentene)] membranes were investigated. The results show that TPX is a highly water permselective material although it is strongly hydrophobic. It was found that, for a symmetric dense TPX membrane, the feed solution vaporizes first and then permeates through the membrane. The water selectivity stems from the huge difference in diffusivity between water and ethanol vapors. To improve the permeation flux, asymmetric TPX membranes were prepared by a wet inversion method. However, due to the swelling effect of ethanol on TPX, small pores occur when the dense skin contacts the feed solution, resulting in a loss of water selectivity. Stain experiments were carried out to verify this mechanism. In addition, it was found that a parallel model can describe the mechanism quite accurately. Good agreement between the theoretical calculation and experimental measurement has been obtained. Furthermore, we also found that the loss of selectivity can be avoided by turning the asymmetric membrane over; that is, let the dense skin face the permeate.