In the case of the pervaporation of methanol and triglyme (triethylene glycol dimethyl ether) through a Nafion membrane, it is shown that concentration polarization can be significant for even nondilute solutions (e.g. 2-4wt% methanol). Concentration polarization was found to be particularly severe for thin composite membranes prepared by casting a modified Nafion solution onto a porous Teflon support. The Grober equation for the liquid-phase mass-transfer coefficient (k(L)) and a resistance-in-series model for the overall mass-transfer coefficient were used to model the boundary-layer effect. The value of k(L) was found to be related to the Reynolds number as 0.145 N-Re(0.5) (units of m h(-1)) for the methanol/triglyme feed stream while the intrinsic membrane permeability was determined to be 2.66 x 10(-6) m(2) h(-1). Using these parameters, the model yielded a reasonable prediction of methanol flux in the feed concentration range of interest.