ZIF-8 crystals with average particle sizes of 14, 23, 65, 144, and 262 nm were synthesized to prepare poly(ether sulfone) based mixed matrix membranes with 10 and 20% loading. The membranes were characterized by measuring single gas permeabilities of H-2, CO2, and CH4 at 35 degrees C with pressures from 3 to 15 bar. The separation performance of PES/pNA/ZIF-8 (10%) with 23 nm crystals was also investigated by separating CO2/CH4 mixtures at 3 and 10 bar. The single gas permeabilities decreased with increasing particle size of ZIF-8; however, the ideal selectivities depended on the ZIF-8 content of membranes. A significant improvement was obtained in the membrane performance when 23 nm ZIF-8 particles loaded PES/pNA(4%)/ZIF-8(10%) MMM. This membrane had a CO2/CH4 selectivitiy of 56.2 and permeability of 1.54 Barrer in the separation of 50% CO2-50% and CH4 mixture at 35 degrees C and 10 bar feed pressure.