A new way to prepare nanofiltration membranes, consisting of in-line external modification of the skin of a polysulfone ultrafiltration hollow fibre skin, is described. The paper presents a continuous process (dip-coating followed by photografting) and the influence of the operating conditions on the membrane characteristics. This study is focused on a simplified model based on the evaluation of the two sources of monomer available for grafting: one is monomer contained in the thin film drawn from the dip bath, second is contained in the pores of the membrane. The results show that two extreme types of modification can be produced, depending on the experimental conditions: a high rate coupled with a short dip contact time leads essentially to an external grafting whereas a low rate coupled with a long dip residence time leads essentially to an "in pore" grafting.