This study investigates the gas separation performance of a mixed matrix membranes flat sheet based on polyethersulfone/polyimide (PES/PI) miscible blend incorporated with zeolite particles. Zeolite 4A was selected and its loading were varied between 10 and 50 wt%. Initial attempt of annealing at 150 and 240 degrees C resulted in ideal separation factors far lower than the intrinsic values of the host polymeric membranes. The ideal separation factor was improved by a factor of 5 by increasing the annealing temperature up to 280 degrees C, which is above the glass transition temperature (T-g) of the polymer blend. Only one Tg was observed in DSC analysis, indicating the high miscibility of PES/PI polymer blend and zeolite particles. The ideal separation factors of O-2/N-2 were higher compared to the value reported for the PES/PI blend membrane and the value increased with increasing zeolite loading. It is concluded that addition of zeolite particles into the matrix of PES/PI polymer blend has significant effect on the membrane structure and properties. (C) 2008 Elsevier B.V. All rights reserved.