In this study, the preparation of high flux gas separation flat sheet membranes based on polyethersulfone (PES) and polyimide (PI) Matrimid 5218 blends was investigated. The process of dry/wet phase inversion with three different compositions of PES/PI (80/20, 50/50 and 20/80 wt%) in N, N-dimethylacetamide (DMAc) or tetrahydrofuran (THF) were employed. The utilization of these solvents has not been reported. Scanning electron microscopy (SEM) was used to investigate the morphological characteristics and the structure of asymmetric membranes. The results indicate that the blend ratio of the polymers plays a vital role in membrane structure and capability. The prepared membranes possessed a typical asymmetric structure with a dense skin layer and a porous substructure. Macrovoids were observed in the membrane substructure with high PES concentration (80 wt%). The effect of PES/PI membrane surface coating with polydimethylsiloxane rubber solution on gas permeance and selectivity was investigated. After coating, the modified membranes exhibited O2 permeances from 3 to 36 gas particulate unit (GPU). However, the membrane selectivity for O2/N2 was decreased from 18 to 5 at room temperature. (C) 2011 Wiley Periodicals, Inc. Adv Polym Techn 31: 298309, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.20253