The transport performances of carbon dioxide (CO2) and methane (CH4) gases were investigated in polyesterurethane mixed matrix membranes (MMMs), in separate tests, containing different fumed silica nanoparticles. Non-modified and commercially-modified silica (with octylsilane and polydimethylsiloxane) were used as fillers. The structural characteristics of the membranes were studied by scanning electron microscopy, infrared spectroscopy, X-ray diffraction and differential scanning calorimetry to confirm some interactions between the PU and nanoparticles and the changes induced in PU microphase in the presence of silica. Surface treatment of silica filler with long hydrophobic chains reduced the aggregation of nanoparticles and improved dispersion in MMMs. Addition of silica nanoparticles into PU matrix increased both separation factor and CO2 permeation in the membranes. The fumed silica disrupted chain packing and increased dynamic free volume to improve CO2 permeation regardless of silica type. The results revealed that among silica nanoparticles, the unmodified particles with OH groups on their surface showed better performance for CO2/CH4 separation. In our current research, a new model for prediction of gas permeation through PU/silica membranes was proposed considering the presence of filler aggregation in the matrix as well as free volume at the interface layer. (C) 2013 Elsevier B.V. All rights reserved