Diffusion of small and medium-sized molecules through UF membranes is an important phenomenon in a number of processes such as membrane bioreactors and blood purification. Numerous theoretical and experimental investigations have been carried out on UF membranes. For various reasons, these studies have mainly been focused on hollow fibers UF membranes. Diffusion through tubular membranes is more complex than that through hollow fibers membranes for several reasons. Hollow fibers membranes are self-supporting; their porous structure can be divided into two layers of the same material : a dense, thin, active layer and a more porous, thick, "supporting" layer. These two layers are typical for the well-known asymmetric porous structure. Polymeric tubular membranes present a more complex porous structure than hollow fibers : a dense layer and a more porous layer (as in hollow fibers); a carrier or fibrous support, and finally a strong mechanical support. This paper presents several aspects of sodium chloride diffusion through polymeric tubular UF membranes : the experimental technique used is briefly discussed; experimental values are given for the mass transfer coefficients of the different layers composing the tubular membrane; and, finally, a general discussion concerning the relative contributions of each layer to the global mass transfer coefficient is presented.