The surface of poly(ethylene terephthalate) (PET) track etched membranes (TMs) was covered with thermo-responsive poly(N-vinylcaprolactam) (PVCL) by a photochemical immobilization method. First, a photo-reactive azidophenyl group was incorporated into the then no-responsive PVCL chains. Secondly, the polymers were cast on the PET membrane followed by irradiation with UV light. Characterization by X-ray photoelectron spectroscopy and FT-ATR-IR demonstrated that the thermo-responsive polymer chains are effectively grafted on the membrane surface. Scanning electron microscopy and atomic force microscopy on the dry composite membranes demonstrated that the pore size diameter decreases and the surface roughness increases with increasing amount of grafted polymer chains on the TM. The water permeation through the modified TM drastically increases when the cloud point (T-cp) of the grafted PVCL-chains is reached. The permeability of a mixture of dextrane molecules is not only controlled by the molecular weight of the individual macromolecules but also strongly by temperature. This last observation opens the possibility to use the same membranes for the separation of macromolecular mixtures over a broad molecular weight range. (c) 2005 Elsevier B.V. All rights reserved.