Hybrid polyimide-siloxane copolymers containing different proportion of silica have been prepared by polycondensation, imidisation and sol-gel coreaction of pyromellitic dianhydride, aminoalkoxysilane and tetramethoxysilane. Two types of aminoalkoxysilane (aminopropyltrimethoxysilane and aminopropylmethyldiethoxysilane) able to provide a bonding between the inorganic and the organic phase were used. Materials obtained were characterized by various techniques (thermogravimetric analysis, infrared spectrometry and Si-29 NMR) in order to study their molecular structure. The presence of methyl side-groups linked to the silicon of APrMDEOS precursors inhibit the formation of OH free or OH linked bonds in the material. Gas permeation measurements were obtained at various temperatures for hydrogen, carbon dioxide and nitrogen. The permeability properties of the two materials vary with the siloxane content and the nature of the aminosiloxane. Thus, the presence of methyl side-groups in the APrMDEOS materials and the network crosslinking are a limiting factor for the gas permeation. The gas transport is thermally activated and the activation energy values vary with the siloxane content of the materials.