The major objective of the work was to develop a heterogeneous modification method for attachment of reactive groups, suitable for covalent immobilization of active biomolecules on the surface of polysulfone ultrafilters without loss of membrane selectivity. For applying a polymer specific activation chemistry, the materials of commercial "polysulfone" UF membranes were identified using elemental analysis along with H-1 NMR, FTIR-ATR and UV spectroscopy. Heterogeneous photoinitiated graft polymerization was realized using acrylic acid (AA) as model monomer and as carrier of reactive groups. Polymer structure (polysulfone, PSf, or polyethersulfone, PESf), coating with photoinitiator (benzophenone, BP, or benzoylbenzoic acid, BPC) and UV excitation energy (lambda(exc) > 220, > 300 or > 350 nm) were the major parameters. Grafted polyAA (g-PAA) could be obtained under almost all conditions but with largely varying yields (DG). However, only with lambda(exc) > 350 nm, polymer and pore degradation could be excluded. A new selective initiation of graft polymerization onto PSf, H-abstraction by photoexcited BP derivatives from thr methyl side groups. thus avoiding polymer chain scission, was proved indirectly. Modified structures were characterized spectroscopically, including visualization with SFM of laterally patterned surfaces generated by UV irradiation through a mask. UF tests of PSf-g-PAA and PESf-g-PAA UF membranes (DG similar to 100...150 mu g/cm(2)), prepared under "mildly degrading" conditions (lambda(exc) > 300 nm), indicated only slight permeability and selectivity changes compared with unmodified samples. Selective PSf functionalization (BPC coating, lambda(exc) > 350 nm; DG 5 mu g/cm(2)) caused flux reductions and dextran selectivity increases by factors of similar to 1.3. Covalent immobilization onto g-PAA functionalized and carbodiimide-activated PSf or PESf membrane surfaces was studied with a protein (BSA), an enzyme (invertase, INV), an antibody-enzyme (IgG-POD) conjugate,ana a peptide ("PC1") as specific antigen of a monoclonal antibody. High binding capacities? up to 40 fold compared with a flat unmodified surface, were detected either directly (BSA) or indirectly via specific activity/binding assays (INV, IgG-POD,"PC1"). This indicated an increased outer membrane surface area due to multifunctional reactive and hydrophilic grafted polymer chains.