The differences in structure and transport mechanisms of nanofiltration (NF) and pervaporation (PV) membranes were studied by measuring (a) water and ethanol fluxes and raffinose rejections obtained during nanofiltration with two NF membranes (Desal 5 DK and MPF 50) and two PV membranes (PV 2201 and PV 1070); (b) water and ethanol fluxes and water/ethanol-n-propanol separation factors obtained during pervaporation with the same membrane sequence; and (c) gas and vapor (water, ethanol) permeance with the same membrane sequence. In the nanofiltration experiments, it was observed that transport is determined by hydrophilicity/hydrophobicity of the membrane, the presence of pores and their geometry, membrane thickness, and solvent properties. Pervaporation experiments confirmed the nanoporosity of the NF membranes; a comparison of fluxes showed that transport in pervaporation is thought to depend on preferential sorption. Gas permeance measurements with inert gases provided clear evidence for the porous nature of NF Desal 5 DK and the dense nature of PV 1070; PV 2201 was found to be porous. However, this membrane swells significantly when brought in contact with water or ethanol vapors, rendering a dense membrane suitable for pervaporation. For the other membranes, swelling was also observed, together with plasticization of the polymers.