A heteroporous membrane model consisting of two membranes (actually suitable to any number of membranes) differing in their separative parameters is advanced. This model has been verified using a number of industrial reverse osmosis and ultrafiltration membranes made of cellulose acetate as well as two nanofiltration ones. The experimental and computed results are in reasonable agreement, particularly taking into account the oversimplified and somewhat mechanistic character of the approach. The results reached clearly and convincingly illustrate the detrimental role which can be played by the presence of even a minor fraction of highly permeable and slightly retentive sections in the membrane structure, that is, the presence of imperfect sites as these often called.