Described in this paper is the synthesis and gas permeability characterization of poly [bis-(4-tert-butylphenoxy)phosphazene], and poly[bis(3,5-di-tert-butylphenox)(1,2)(chloro()0.8)phosphazene]. In general, linear chloro-containing polyphosphazenes are hydrolytically unstable. However, in this work, a novel polymer, poly[bis-(3,5-di-tert-butylphenoxy)(1.2)(chloro)(0.8)phosphazene], was observed to have an unusually high degree of hydrolytic stability and excellent membrane formation characteristics. Data derived from these polymers were compared to that of the more common poly[(bis-phenoxy)phosphazene]. These comparisons showed higher gas permeabilities and ideal separation factors for both of the alkyl-substituted phenoxy-phosphazenes, thus validating the concept that adding sterically bulky pendant groups to phosphazenes can affect membrane performance through disruption of orderly chain packing. Chemical characterization of these polymers was conducted using NMR spectroscopy, thermal analysis, helium pycnometry, elemental analysis, and multi-angle laser light scattering. Membranes were formed by solution casting and were characterized for their pure gas permeability using the following gases: H-2, Ar, N-2, O-2, CH4. CO2, and H2S. Additionally, ideal selectivities of the significant O-2/N-2 and CO2/CH4 gas pairs are discussed. (C) 2004 Elsevier B.V. All rights reserved.