AbstractThe structure of cellulose acetate membranes used for desalination in the process of reverse osmosis has been examined. By means of a technique involving aqueous solutions of the stain Biebrich Scarlet, the membranes are shown to consist of two regions, one of which acts as a mechanical support for the much thinner desalinating layer. The phenomenon of asymmetric permeation across these membranes has been studied and shown to be a consequence of the two‐layer structure of the membran