The morphology together with the gas permeation properties of crosslinked membranes prepared from different amounts of a poly-organophosphazene -(-NP(R-1)(0.18)(R-2)(0.76)(R-3)(1.06)-)n- where R-1 =2-allylphenoxy, R-2 =4-isobutylphenoxy and R-3 =4-methoxyphenoxy ((M) over bar (w) = 1, 350, 000 g/mol) and a hydride terminated polydimethylsiloxane Mn = 14, 400 g/mol were investigated. The microstructure of some resulting films was first examined by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The transport properties of the films were second examined with respect to their permeability to light gases such as He, H-2, O-2, N-2 and CO2, and selectivity ratios were established. Different theoretical copolymer permeability models were tested as a function of the chemical composition to investigate the relationship between morphology and permeation properties and to give a complete insight into the copolymer morphology. Selectivity ratios are discussed as a function of the penetrant nature and of the copolymer structure. (c) 2006 Elsevier B.V. All rights reserved.