The hydrodynamics and concentration polarization in the feed-channel of a NF/RO spiral-wound module with ladder-type spacers were investigated by computational fluid dynamics. The momentum and mass transport equations together with the appropriate boundary conditions were solved numerically by the control volume formulation for stable two-dimensional laminar flow. Permeation experiments with aqueous solutions of sodium chloride at feed concentration of 2 g/l and 25degreesC were performed in a laboratory NF cell with a spacer-filled channel (2 min height x 30 min width x 20 cm length) in order to validate the numerical model. The tested spacer had a set of transverse filaments with a diameter of 1.0 mm, equally spaced and connected by two longitudinal filaments, with a distance between the axis of two consecutive filaments of 3.8 mm, forming a ladder-type structure. A thin-film composite nanofiltration membrane from Separem (Italy) was used. The numerical results show that the concentration polarization index exhibits strong local variations, and its profile is correlated with the flow pattern. It was also found that the increase of the, Reynolds number is not by itself a sufficient condition to enhance the hydrodynamic conditions over the whole membrane: an adequate control of the flow structure through the careful selection of the cross section of the filaments is also indispensable. Good agreement was observed between the predicted and experimental values of the apparent rejection coefficient for all the range of operating conditions tested.