Three phosphazene polymers were characterized for permeability using a suite of pure gases, including H2S where high permeabilities were measured with respect to the other gases in the study. Mixed gas selectivities were determined and compared to the ideal gas selectivities for the H2S/CH4, CO2/CH4, and Ar/CH4 gas pairs. The three phosphazenes represent a set of membrane materials differing by their polarities. Description of the polarity of each was performed using Hansen solubility parameters derived from group contributions for each chemical structure. A good correlation was observed between the polar Hansen parameter (delta(p)) and the gas permeabilities of both CO2 and H2S. Furthermore, permeant gas critical temperatures were also found to correlate with permeability, with the exception of He and H-2. A comparison of the mixed gas permeabilities with the corresponding pure gas data revealed good agreement in the data. Most surprisingly, permeabilities and separation factors were observed to increase with decreasing glass transition temperature (T-g) for the H2S/CH4 and CO2/CH4 gas pairs reaching a maximum of P-H2s = 1103 x 10(-10) cm(3) (1103 Barters) with and alpha(H2S/CH4) = 78.8 as derived from a mixed gas experiment. (c) 2005 Elsevier B.V. All rights reserved.