Poly (ether sulfone) (PES) 50 kDa membranes were surface modified by irradiation with UV light (254 nm) in the presence of N-vinyl-2-pyrrolidine (NVP), 2-acrylamidoglycolic acid monohydrate (AAG) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AAP). The surfaces of the modified membranes were chemically characterised using X-ray photoelectron spectroscopy (XPS) and time of flight-static secondary ion mass spectrometry (TOF-static SIMS). The filtration performance of irradiated/non-modified and irradiated/modified membranes was examined in a crossflow cell, using a dextran solution. The filtration performance of the irradiated/non-modified membranes unambiguously indicates that cross linking and chain scission to the base membrane is occurring. The simultaneous decrease in volume flux and true retention demonstrates that the proceeding cross-linking increases the hydrodynamic resistance of the membrane while chain scission is responsible for loss of membrane selectivity. The presence of monomer during UV-irradiation seems to retard loss of membrane selectivity compared with membranes irradiated in the absence of any monomers at the same energy density. NVP modified membranes exhibit higher retention in relation to dextran when compared to membranes modified by AAG and AAP. This work suggests that the structure of the presence of grafted chains seems to be responsible for the observed changes to filtration performance of the modified membrane. Surface analysis supports the claim that the specific surface chemistry and structure plays an important role. In particular, NVP is markedly different from surfaces generated from the two other monomers.