Gas permeabilities and selectivities of some gases (He, O-2, N-2, CO2) were evaluated in inorganic-organic membranes based on 6FDA-6FpDA and 6FDA-6FpDA:DABA and PMMA-g-silica. PMMA was grafted onto the silica to make it miscible with 6FDA-based polyimide after surface treatment with gamma-MPS. The PMMA-g-silica showed greater dispersion than did the pure silica particles in the polymer matrix. The two types of polyimide polymer had similar trends with respect to gas transport properties. The increase in permeation was attributed to changes in the free volume distribution up to 1 wt% silica. Above 1 wt%, the permeabilities decreased as a result of reduced effective area for gas transport. The selectivities decreased when PMMA-g-silica content increased. However, the selectivity of CO2 in 6FDA-6FpDA was enhanced because of a strong interaction between PMMA and polar gas. (C) 2011 Elsevier B.V. All rights reserved.