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Two types of POSS (polyhedral oligomeric silsesquioxane)-polymers, (POSS-polyimide-phenyl (POSS-PI-Ph) and POSS-polyimide-phenyl-norbornene (POSS-PI-Ph-Norbornene)), were utilized for the fabrication of highly permeable gas-separation membranes. POSS-PI-Ph and POSS-PI-Ph-Norbornene separation layer for selective gas separation were successfully formed on porous intermediate layer. A POSS-derived membrane calcined at 250–350 °C under N 2 showed approximately the same levels of selectivity for He/N 2 and CO 2 /N 2 as conventional polyimide membranes, which shows there was no formation of large pores that decreases gas selectivity. The introduction of norbornene, which forms a cross-linked structure by thermally inducing cross-linking at 500 °C under N 2 , improved gas permeance without decreasing the gas-permeance ratio. A POSS-PI-Ph-Norbornene membrane heat treated at 500 °C showed increases in He permeance that ranged from 1.2 × 10−7 to 4.6 × 10−7 mol m−2 s−1 Pa−1 with He/N 2 , CO 2 /N 2 , He/CF 4 , and He/SF 6 permeance ratios of 38; 8; 2700; and 14000, respectively. The relationship between the activation energy of H 2 permeation and the He/H 2 permeance ratio in POSS-derived, organosilica, and polymer (polyimide, polyamide, polysulfone) membranes suggests that gas permeation properties depend on the concentration of organic groups (C/Si ratio) in networks, because networks become more flexible when carbon concentration increases and dominates the permeation properties, which corresponds to a solution-diffusion mechanism. Image 1 • Gas separation membrane was fabricated by POSS containing polyimide polymer. • Thermally induced crosslinking of norbornene was effective for improved thermal stability. • Gas permeation properties depended on the concentrations of the organic groups in the networks. [ABSTRACT FROM AUTHOR]