A two-step heterogeneous polymer-surface modification was developed, involving photo-bromination as activation and subsequent UV-induced graft polymn. with acrylic monomers. With polyacrylonitrile (PAN) ultrafiltration (UF) membranes, extent and effects of photo-bromination compared with the thermal reaction were studied with gravimetry, diffuse-reflectance UV and ATR-IR spectroscopy, and SEM as well as UF properties. The yields of thermal and photo-bromination reached const. values of about 60 and 150 mmol/g, after 24 an 3 h, resp. The former Br content was assigned to Br2 addn. to double bonds, the latter to Br-radical CH substitution plus addn. products. Excess bromine was bound as a charge-transfer complex to PAN nitrile groups, causing partially irreversible changes of the membrane matrix indicated by UF permeability redns. The formation of polymer radicals after UV irradn. of photo-brominated PAN was verified with ESR spectroscopy. UV irradn.-induced graft polymn. of acrylic acid and Me acrylate from the gas phase onto PAN membranes was accomplished. It was found that activation by thermal bromination is sufficient to initiate heterogeneous graft polymn. Sorbed bromine promoted homopolymn., increasing concns. of hydroquinone and improving the grafting efficiency. The graft-polymer modified surfaces were characterized by ATR-IR, SEM and contact angles. A pronounced selectivity towards the UV-exposed membrane surface was achieved. For relatively low degrees of modification (up to 150 mg/cm2), thin and smooth graft polymer layers were created which specifically altered the membrane-surface hydrophilicity. UF membrane permeability and selectivity were a direct function of the degree of modification, suggesting adjustability of av. sepn.-layer pore size.