Two series of mixed matrix membranes were prepared by adding three organic polymeric fillers into polyimide matrices to explore the effect of the filler on the membrane preparation, properties and gas separation performance. The fillers were two mesoporous polyimides, with specific surface areas of 605 and 715 m(2)/g, and a microporous one. The polymer matrices used in this study were a commercial polyimide, Matrimid (R), and a synthetic polyimide. Due to the chemical affinity between the fillers and the matrices, very good and quick dispersion was achieved in all cases. The spectral characteristics and the thermal properties of the mixtures did not greatly differ from those of the pristine polyimides, as it was checked by FT-IR, TGA and DSC. The gas separation properties of Matrimid (R) were significantly improved when using the mesoporous fillers because the permeability coefficients were increased up to 3.4 Limes while the selectivity was maintained or even exceeded. However, when the synthetic matrix, which has much more permeability than Matrimid (R), was used, no significant increases in these parameters were observed upon incorporating the porous fillers. (c) 2013 Elsevier B.V. All rights reserved.