The molecular-level mixed matrix membranes (MMMs) comprising Pebax (R) and FOSS have been developed by tuning the membrane preparation process in this work. They exhibit a simultaneous enhancement in CO2 permeability and CO2/H-2 selectivity by optimizing the FOSS content at extremely low loadings. This is mainly attributed to the large cavity of FOSS itself and its effect on the segmental-level polymeric chain packing. More interestingly, the Pebax (R)/POSS MMMs reveal a much higher separation performance in the mixed gas test than that in the pure gas test. The highest CO2/H-2 selectivity reaches 52.3 accompanied by CO2 permeability of 136 Barrer at 8 atm and 35 degrees C. This is due to the CO2-induced plasticization that improves the free volume and polymer chain mobility, hence benefiting the interaction between the polymer matrix and penetrant CO2. These features may ensure the superiority of Pebax (R)/POSS molecular-level MMMs as CO2-selective membranes in the industrial application of hydrogen purification. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.