© 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.A generalized course of action was proposed for determining the separation properties of the interphase between filler and polymer phases in various mixed matrix membranes (MMMs). Accordingly, the permeability and thickness of the interphase were estimated by using the modified form of effective medium theory (EMT) and CO2, N2 and CH4 permeability data at two different filler loadings obtained from the literature. In spite of the sensitivity of the estimations to experimental inconsistencies in the data used, significant information could generally be obtained about the interphase permeability and thickness. It was observed that the interphase in different types of MMMs could have dissimilar separation properties even when the polymer used was the same. The CO2/CH4 ideal selectivities of the interphase between graphene oxide and polyether block amide (Pebax) polymer and the interphase between zeolite Y and a polyimide polymer, Matrimid, were determined to be higher than the selectivities of the pure polymers. Tailoring the interphase by having good knowledge of its properties, is expected to provide significant means for further improvement of the performances of mixed matrix membranes.