Heterogeneous surface modification of polyacrylonitrile (PAN) ultrafiltration (UF) membranes is realized with UV irradn.-initiated graft polymn. of acrylic acid (AA) from the gas phase onto photoinitiator (benzophenone, BP)-coated samples. In the absence of monomer, PAN functionalization by ketyl radicals dominates after UV excitation of sorbed BP. With AA, graft and total polymer yield increase with BP loading with UV irradn. time. Av. mol. wt. and distribution of PAA homopolymer, formed in parallel during graft polymn., are analyzed with gel permeation chromatog. Morphol. of PAN-gr-AA UF membranes is checked with scanning electron micrographs (SEMs) and at. free microscopy. Chem. changes are characterized with FTIR-ATR spectroscopy and SEM/EDX analyses, indicating a pronounced surface selectivity of the graft polymer modification (localized in the upper 5 mm membrane thickness). The amt. of grafted PAA systematically reduces membrane permeability and increases dextran retention, as verified in UF expts. Photo grafted polymer modification of UF membranes will be applied to adjust membrane performance by controlling surface hydrophilicity and permeability using other monomers and/or further graft polymer functionalization.