Mixed matrix membranes of polyethersulfone (PES), a glassy polymer, and hydrophilic zeolites 13X and 4A were prepared by using different membrane preparation procedures. Using selected procedure (c), the permeation rates of N2, O2, Ar, CO2 and H-2 were measured with a variety of membranes prepared at different zeolite loadings. Significant differences in measured permeability and calculated selectivity values demonstrated the importance of the type and percentage of zeolite. For both zeolitic additives, permeabilities and selectivities are enhanced at high zeolite loadings. In order to understand the permeation mechanism taking into account the polymer-zeolite interactions, macro-positioning of zeolites and matrix distribution, the heterogeneous membrane morphologies were investigated by scanning electron microscopy (SEM). Significant changes in the membrane morphologies of PE-13X and PES-4A matrices were observed, implying the importance of zeolite type.