Polymeric blend mixed matrix membranes (PB3Ms) are synthesized with different blend compositions at a fixed content of carbon molecular sieve (CMS) inorganic filler. The resulting membranes are characterized by FESEM, TGA, DSC and FTIR analyses. Polyethersulfone (PES)/polyvinyl acetate (PVAc)/CMS polymeric blend mixed matrix membranes are prepared by solution casting method. A homogenous mixture of all these components is formed and cast on a clean glass plate. The produced PB3Ms show drastic enhancement in CO2 permeance and CO2/CH4 selectivity in both single and mixed gas permeation tests in comparison with pure PES membrane. Significant CO2 permeance of 132.47 GPU with CO2/CH4 selectivity of 75.27 at 6 bar is obtained for the PB3M having 80 wt/20 wt blend ratio of PES/PVAc and 20 wt CMS loading. The synthesized polymeric blend mixed matrix membranes in this research study demonstrate competitive CO2/CH4 separation performance in comparison with the reported literature on PB3Ms for gas separation. © 2021, Institute of Chemistry, Slovak Academy of Sciences.