The intrusion of a polymer solution into microporous substrates may have a detrimental effect on the performance of ail-separation membranes. An extremely simple equation has been derived in this paper for the calculation of intrusion depth in a multi-layer microporous composite membrane. A modified resistance model has also been proposed in order to calculate the intrusion effects on microporous composite membrane performance. When comparing the calculated results to the previous experimental data on 6FDA-polyimide/polyacrylonitrile (PAN) hollow fibers, the new model accurately predicts the air-separation performance of multilayer microporous composite membranes. However, this model is only valid if the surface-porosity ratio multiplying the permeability difference (between the gutter and the substrate) is much greater than the substrate permeability. (C) 1997 Elsevier Science B.V.