Pervaporation experiments with flat sheet PDMS-PVDF composite membranes were carried out at ambient temperature (20-22degreesC) to concentrate three flavor compounds (ethyl butyrate, benzaldehyde, and trans-2-hexenal) from dilute aqueous solutions. Due to the different thermodynamic properties (saturated vapor pressure and activity coefficient) of these flavor compounds, the effects of downstream permeate pressure on the organic flux and the enrichment factor varied. The results were correlated with simple mass transfer equations. The organic and water fluxes at different feed flow rates were measured. The feed flow rate had negligible effect on the water flux because of the dilute solute concentration in the feed solution. However, for the organic flux, the significant effect of the feed flow rate proved the existence of a mass transfer boundary layer on the membrane feed side. A concentration polarization model derived from a steady state flux equation was used to calculate the liquid boundary layer thickness on the feed side and the organic mass transfer coefficient in the membrane. Using the resistance-in-series model, the mass transfer resistance of the liquid boundary layer was separated from the mass transfer resistance of the membrane. It was clearly shown in this work that the mass transfer resistance of the boundary layer was controlled by hydrodynamic conditions, while the varying degrees of flavor concentration results were mainly due to the different thermodynamic properties. (C) 2004 Elsevier B.V. All rights reserved.