Abstract Gas permeability coefficients of poly(bis-trifluoroethoxyphosphazene), (PTFEP) and its blends with adamantane amino/trifluoroethoxy polyphosphazene (PAdATFEP) containing adamantane groups on 50 mol% of the side chains were determined at 35°C. The PTFEP films prepared by both spin casting and solvent casting had the same permeability coefficients, within the precision of the measurements. PTFEP/PAdATFEP blends were phase-separated. Relative to pure PTFEP, gas permeabilities were reduced from 45 to 70% (depending on the penetrant) by blending 18.8 mol% of PAdATFEP with PTFEP. The permeability coefficients of low-sorbing, permanent gases (i.e. H 2, O 2, and N 2) decreased as penetrant size increased, suggesting that such polymers sieve these permanent gas molecules primarily based on penetrant size rather than penetrant solubility. Gas permeation properties of CO 2 and hydrocarbon vapors did not change monotonically with penetrant size, suggesting an interplay of gas diffusivity and gas solubility on overall gas permeability. Blending PAdATFEP with PTFEP increased gas selectivity. Ideal separation factors of these PTFEP/PAdATFEP blends were significantly below upper bound values.