Phosphazene polymers are unique materials in that series of materials can be synthesized with a uniform backbone structure while varying the pendant group speciation. The result of this variation is that the physical properties can be controlled through adroit selection of pendant groups. in this work, a series of phosphazenes have been synthesized using phenol and 4-phenylphenol (PP) as the pendant groups. The relative amounts of PP were varied between 10% and 47%. As the PP was increased, the glass transition also increased. Furthermore, at 10.0% and 47.4% loadings. the polymers were found to be semicrystalline. Loadings in between these values, 25.9%, 30.6%, and 34.5%, were found to be completely amorphous. Gas permeability characterization of these materials showed that, regardless of the crystallinity, increased permeability with decreasing PP loading. Gases used in this study include O-2, N-2, CO2, CH4, He, and H-2. Additionally, ideal separation factors for the gas pairs O-2/N-2, He/N-2,N- and CO2/CH4 are discussed. (C) 2009 Elsevier B.V. All rights reserved.