Transport in mixed matrix membranes is affected significantly by the adherence between the matrix and the inclusions that are added to improve the membrane selectivity. The role of the matrix/inclusions interphase layer in the species transport is investigated in this work through a two-dimensional model structure that allows free overlapping of the interphase layer around the inclusions. The mean size of the inclusions is found to affect the overall permeability in a fashion that follows relevant experimental observations. The transport rate through the inclusions is also calculated and the degree of utilization of the inclusions is discussed as a function of the volume fractions of inclusions and interphase layer, the size of the inclusions, and the permeability values for the three phases. A comparison of the results for uniformly sized inclusions and for particle sizes that follow a lognormal distribution reveals that both the effective permeability and the flux through the inclusions are essentially the same for the two types of inclusions. This implies that the simplified unimodal particle size calculations suffice to provide estimates of the transport rate through the membrane and can be used for the design of mixed matrix membranes with maximized utilization of the sieving properties of the inclusions. The accuracy of effective medium approximations that are typically used in the mixed matrix membrane literature to predict the overall permeability is assessed through comparison with the exact numerical results, and alternative homogenization processes and simplified expressions are suggested. (C) 2012 Elsevier B.V. All rights reserved.