Concentration polarization dominates the separation of dissolved volatile organic compounds from water by pervaporation. This is particularly true with hydrophobic organics, such as toluene and trichloroethylene, for which concentration polarization is severe even in highly turbulent membrane modules. With these compounds, measured separation factors can be 10 to 20% of the intrinsic separation factors in the absence of concentration polarization. As a result of concentration polarization, unexpected permeation properties are observed. For example, the organic flux is independent of membrane thickness over a wide range, whereas the water flux decreases with membrane thickness. Consequently, thicker membranes are preferred over thinner ones. Also, the organic flux is relatively independent of permeate pressure over a wide range, whereas the water flux decreases as the permeate pressure increases. This means that the separation performance improves as the driving force across the membrane decreases, contrary to normal membrane behavior. These and other consequences of concentration polarization are described in this paper.