Polyphosphazenes are a class of inorganic polymers characterized by the phosphorus nitrogen repeating unit that forms the backbone. The phosphorus is pentavalent and the backbone has alternating single and double bonds. This leaves two coordination sites on the phosphorus free for substituted with a variety of nucleophilic groups. Several hundred polyphosphazene formulations have been reported in the literature. The ease of controlling the type and number of substituents provides a unique opportunity to develop special series of polymers to investigate structure property relationships. In this paper the pure gas permeabilities of a series of substituted bisphenoxyphosphazene polymers is reported. The polymers were exposed to ten different gases and the resulting permeabilities were analyzed using the time lag method. The time lag method enables the permeability to be broken down into its solubility and diffusivity components. Careful examination of the results makes it possible to determine what types of substituent-gas interactions are responsible for the overall permeabilities in the polymers. Some of the polymers showed low permeabilities for all of the test gases, while others showed very large differences among the different gases tested. Especially interesting differences were observed for the series of alkanes tested. The results permit the prediction of what types of mixed gas separations might be worth pursuing.