Mixed matrix membrane materials (MMMs) are gaining much interest in the field of gas separation research. This work investigates the ‘right’ filler and matrix combination (in terms of phase gas transport compatibility) in an MMM to achieve maximum selectivity in gas mixtures. Through rigorous computational simulations, it has been found that by adding a filler with comparable permeability of the faster gas to the polymer matrix can result in maximum change in selectivity in the matrix. These results have also been compared with the analytical solutions by various models like Maxwell model. This work also examines the role of interfacial defects in altering the transport properties of mixtures through the MMMs.