The impact of the solubility partition coefficient at the matrix-filler interface on the effective diffusivity of mixed matrix systems has been calculated using effective medium theory. These predictions are able to explain trends in mixed matrix membrane performance reported in the literature. In particular, partition coefficients which favor sorption of the penetrants in the matrix phase over the dispersed phase, will lead to an increase in the selectivity of the MMM compared to that of the matrix, but will decrease the effective diffusion of the penetrant. On the other hand, partition coefficients, which favor sorption in the dispersed phase have the potential to enhance both selectivity and diffusion, although, to observe this particular effect, the volume fraction of the dispersed phase in the MMM needs to be above a certain threshold. (c) 2012 Elsevier B.V. All rights reserved.