The resistances-in-series model, the modified solution-diffusion model, the Flory-Rehner theory, and the film theory were used to calculate the diffusion coefficients of two components of a liquid-feed mixture that are separated by pervaporation. The toluene and water fluxes through EPDM membranes of various thicknesses were modeled for different mass-transfer coefficients in the feed boundary layer (k(L)). Ir is shown that the toluene flux depends strongly on the k(L) and the membrane resistance affects significantly the toluene flux only for very high values of k(L) ( > 10(-4) m/s). The water flux increases linearly with the reciprocal value of the membrane thickness and is not affected by the boundary-layer resistance. The small increase in the water flux, due to the toluene concentration increase in the feed can adequately be described by this model.