The lack of interfacial adhesion between the polymer and inorganic phases in mixed matrix membranes (MMMs) imposes a serious challenge. This study is focused on the improvement of polymer-filler adhesion via surface modification of carbon molecular sieve (CMS) by nitric acid (HNO3) oxidation process. The bare CMS and oxidized CMS based mixed matrix membranes fabricated by the solvent evaporation method. Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analyses confirmed the oxidation of CMS surface. An improved morphology and interfacial adhesion were observed in Polysulfone (PSU)/Ox-CMS MMMs as deduced by field emission scanning electron microscope analysis. Thermal stability of the developed membrane has been confirmed by Thermogravimetric analysis (TGA). Raman spectroscopy is carried out to evaluate the homogeneity of the filler distribution in the polymer phase. Gas separation performance of the synthesized MMMs has revealed the positive effects of surface modification of CMS particles with HNO3 oxidation on CO2/CH4 selectivity of the developed MMMs. A slight reduction in CO2 permeability (2�5) observed in ox-CMS based MMMs. PSU/oxidized-CMS MMM with 30 wt CMS loading has shown a superior performance (CO2 permeability = 3.91 barrer and CO2/CH4 selectivity = 65.92) which is approaching the Robeson upper bound limit. These highly selective membranes have a high potential in membrane-based CO2 separation applications. © 2019 Elsevier Ltd