ABSTRACT The surface modification of a PES hollow fiber by UV photografting has been investigated in order to graft a dense polymer layer. The study focused on a UV photografting process, starting from a monomer solution, enabling the thickness and regularity of the grafted polymer to be followed. 2‐(Acryloyloxy)ethyl trimethylammonium chloride was polymerized on the surface of the PES membrane. Modified membranes were characterized by SEM, FTIR spectroscopy, and liquid and gas permeability. A dense layer of poly(2‐(acryloyloxy)ethyl trimethylammonium chloride) was obtained when a photoinitiator and a photocrosslinker were used. Polymerization of the ammonium material also occurred inside the pores of the membrane. With pretreatment and an increase of the irradiation time, the thickness of the grafted polymer decreased and gas permeability reached measurable levels. However, a CO2/N2 selectivity of around 1 was found which suggested the presence of defects in the grafted layer. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41514.