The diffusion of low molecular weight penetrants through films of a PMDA-ODA polyimide and a corresponding polyimide-silica. bicontinuous nanocomposite (organic-inorganic hybrid) was monitored using time-resolved FTIR spectroscopy and gravimetric analysis. A reacting and a non-reacting penetrant, ammonia and water vapour, were used in the study in order to evaluate their relative interactions with the components of the polyimide-silica hybrid from sorption and diffusion measurements. In the experiments with water vapours the sorption equilibrium was measured with activity levels varying from 0.1 to 0.75. The state of water molecules was examined on the basis of a simplified H-bonding association model. From the experiments using ammonia as the penetrant, it was possible to separate the physical diffusion component from the reaction-determined transport rate by using different sample thickness and penetrant pressure. A remarkable catalytic effect of the silica phase on the reaction of ammonia with the polyimide was observed. © 2005 Elsevier Ltd. All rights reserved.