Organic-inorganic materials comprising CO2-philic components may yield superior CO2 transport properties and good CO2/H-2 gas selectivity. We report that a fine balance in size heterogeneity in the silicon-based structures is essential and a mixture of sizes up to 50 nm surrounded by 5-15 nm silicon-based nanostructures is the preferred inorganic phase morphology that yields optimal nanohybrid membranes. The combination of optimal synthesis conditions i.e. water/silicon ratio, condensation and ozone pre-treatment durations yields a nanohybrid membrane with a CO2 permeability of 2000 Barrer while achieving a CO2/H-2 selectivity of 11. The findings of this work are important for the design of gas separation membranes using green materials. (C) 2011 Elsevier Ltd. All rights reserved.