An octahedral metal organic framework (MIL-101(Cr)) with a uniform size of similar to 550 nm was synthesized via hydrothermal reaction, and then was functionalized with sulfonic acid groups by concentrated sulfuric acid and trifluoromethanesulfonic anhydride. Mixed matrix membranes (MMMs) were fabricated by incorporating the as prepared metal organic frameworks into sulfonated poly(ether ether ketone) (SPEEK). The gas separation performance of MMMs was investigated both in dry and humidified state. The addition of the sulfonated metal organic framework increased the selectivity of the membranes for CO2/CH4 and CO2/N-2 systems by increasing the CO2 solubility, and the diffusion of gases through the porous metal organic frameworks led to the simultaneous increase in CO2 permeability. The highest ideal selectivities for CO2/CH4 and CO2/N-2 were 50 and 53 (at a CO2 permeability of 2064 Barrer) in humidified state, respectively. The increased total water in MMMs led to increased CO2 permeability and the increased bound water resulted in the improved CO2/gas selectivity. The effects of operating conditions on separation performance were investigated. (C) 2015 Elsevier B.V. All rights reserved,