A new application of the 'pore filling' concept yielded high-performance composite membranes for the selective pervaporation (PV) sepn. of org. mixts. Sym. polyacrylonitrile (PAN) membranes (av. pore sizes of 7 or 12 nm) were used as matrix for polymeric PV sepn. phases which were in situ prepd. by heterogeneous photoinitiated graft copolymn. By this means, defect-free and stable layers were synthesized from various moderately hydrophilic (meth)acrylates, e.g. poly(ethylene glycol) (meth)acrylates. The impact of (meth)acylate side group functionality (hydrophilicity, size) and prepn. parameters (monomer concns., UV irradn. time) was analyzed using PV methanol removal from less polar hydrocarbons (cyclohexane or MTBE) as example. High selectivities (amethanol/cyclohexane ? 2000) and extraordinarily high permeate fluxes (J ? 8 kg/m2h) were achieved. Major reasons for the excellent performance were the small effective PV barrier thickness (<1 mm; detd. by the thickness of the selective layer of the matrix membrane) and the fixation of the graft polymer (covalent anchoring, suppressed swelling) in a matrix (PAN) which is not swollen under PV conditions. Due to the functionalization principle, a great variety of thin-layer composite membranes can be prepd., 'tuning' the PV selectivity by functionality of the graft polymer, thus providing possibilities to efficiently solve various PV sepn. problems.