Mixed matrix membranes (MMMs) were developed by incorporating inorganic silica nanoparticles into blends of polysulfone/polyimide (PSF/PI) asymmetric membranes for gas separation using phase inversion technique. The membranes were characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA). SEM results show different morphologies of surfaces and cross-sections of the membrane where agglomeration is observed at 20.1 wt. silica loading. TGA analysis indicates good thermal stability of the hybrid membranes. Permeation results show that CO 2 permeance increased with the introduction of 5.2 wt. silica contents (73.7 ± 0.2 GPU) in PSF/PI-20 blend and it increased with the increase in silica contents. With 15.2 wt. silica contents, the highest permselectivity of αCO 2/CH 4 = 61.0 ± 0.3-60.2 ± 0.4 is observed for treated membrane at 2-10 bar. The selectivity using mixed gas test at various CO 2/CH 4 compositions shows consistent results with the ideal gas selectivity. © 2012 Elsevier B.V. All rights reserved.