Hybrid organic/inorganic films have been prepared using different complexes soluble in a (BTDA/4,4'-ODA) poly(amic acid) solution. A silver based complex and two palladium based complexes were used and the metallic nanoparticles were formed by a single step process during the cure cycle applied to the film. Depending on the complex or the curing conditions used, either the nanoparticles formed were uniformly dispersed in the nanocomposite films, or the particles were also located at the film surface. The presence of the nanoparticles, whatever their composition, led to a decrease of the thermal degradation temperature in air atmosphere. The presence of 15 wt% of crystalline silver particles did not induce significant variation of the glass transition temperature and of the gas transport properties demonstrating low nanoparticle/matrix interactions. On the contrary, a pronounced effect in reducing the gas permeability for a wide variety of gases was observed for the nanocomposites containing the palladium based nanoparticles and this even for low palladium amount (5 wt%). Specific interactions between hydrogen and the Pd based nanoparticles were evidenced and the gas transport properties were discussed as a function of following parameters: the gas nature, the nanoparticle structure and composition, the developed morphology and the particle/matrix interactions. (c) 2006 Elsevier Ltd. All rights reserved.