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 degrees C under N-2 showed approximately the same levels of selectivity for He/N-2 and CO2/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 degrees C under N-2, improved gas permeance without decreasing the gas-permeance ratio. A POSS-PI-Ph-Norbornene membrane heat treated at 500 degrees C showed increases in He permeance that ranged from 1.2x10(-7) to 4.6x10(-7) molm(-2) s(-1) Pa-1 with He/N-2, CO2/N-2, He/CF4, and He/SF6 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.