We use the finite element method to analyze the role of filler aspect ratio and volume loading on the effective permeability and selectivity of gas-separation membranes consisting of a polymer matrix filled with platelet-shaped molecular sieving particles. On the basis of direct 3D finite element estimates, we develop and validate a quick arithmetic procedure for predicting the effective permeability and selectivity of platelet-filled systems. We use this procedure to show that it is feasible to obtain mixed matrix platelet-filled membranes with effective perm selectivities considerably exceeding the upper Robeson's bounds. (c) 2008 Elsevier B.V. All rights reserved.