Permeation models for mixed matrix membranes (MMMs) are discussed. A new model is proposed for the effective permeability of a species in MMMs. The model takes into account the presence of interfacial layer (shell) at the surface of the core filler particles. According to the proposed model, the relative permeability (Pr) of a species in MMM, defined as permeability in MMM divided by matrix permeability, is a function of five variables, namely: ratio of interfacial shell-to-core particle radii (delta), ratio of interfacial shell-to-matrix permeabilities (lambdaIm), ratio of core particle-to-interfacial shell permeabilities (lambdadI), volume fraction of composite core-shell particles (phi), and maximum packing volume fraction of particles (phim). The predictions of the model are discussed and compared with available experimental data on permeability and selectivity of mixed matrix membranes.