The gas permeability, diffusivity, and selectivity properties of gases such as He, O-2, N-2, CH4, and CO2 were evaluated for a series of hybrid inorganic-organic composites based on 6FDA-6FpDA and 6FDA-6FpDA-DABA polyimides and various organo-silica structures. The organo-silica domains were introduced into the polymer matrix via sol-gel reactions. The gas transport properties of these hybrid membranes were dependent on the type of alkoxide employed and the final morphology of the hybrid composite. Systems with low degree of cross-linking between the inorganic and polymer components exhibited membrane performance below the upper bound. However, simultaneous improvements in selectivity and permeability were observed in hybrid materials with favorable interactions between inorganic and polymer domains as documented by changes in viscoelastic properties, These improved interactions most likely led to the formation of an interphase consisting of constrained polymer segments partially incorporated into the silicate structure.