In this study, the effect of TiO2 nanoparticles addition on mixed matrix membranes (MMMs) based on Matrimd5218 prepared by using solution-casting method has been investigated. The morphology of MMM was evaluated by Scanning Electron Microscopy (SEM). TiO2 nanoparticles distribution in matrimid matrix and top surface of membrane films was observed by Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM), respectively. It was found that at low particle loadings (less than 15%) nanoparticles were mainly dispersed individually forming nanoscale aggregates. However, at high volume fractions (more than 20%) the size of aggregates increases and micro-scale aggregates form. The permeability of N-2, CH4, O-2, CO2 and He have been measured in order to determine the effect of TiO2 on gas separation performance of Matrimid5218. The results demonstrated that inclusion of TiO2 increases the gas permeability of MMMs possibly due to chain packing disruption, void formation at polymer-nanoparticle interface as well as within nanoparticle aggregates. For example, in MMMs containing 15 vol.% TiO2. the permeability of N-2. CH4 and CO2 increased up to 2.76, 3.3 and 1.86 times higher than pure matrimid respectively. Diffusivity coefficients of larger molecules obtained by time-lag method show that permeability enhancement is mainly affected by diffusivity increment. Finally, the results reveal that use of TiO2 nanoparticles improves membrane performance in CO2/CH4 separation and presents a trade-off line with similar slope compare to Robeson upper insertion bound. (C) 2010 Elsevier B.V. All rights reserved.