An interfacial polymerization process is introduced for the fabrication of thermally stable cyclomatrix poly(phenoxy)phosphazenes thin-film composite membranes that can sieve hydrogen from hot gas mixtures. By replacing the conventionally used aqueous phase with dimethyl sulfoxide/potassium hydroxide, a variety of biphenol molecules are deprotonated to aryloxide anions that react with hexachlorocyclotriphosphazene dissolved in cyclohexane to form a thin film of a highly cross-linked polymer film. The film membranes have persistent permselectivities for hydrogen over nitrogen (16–27) and methane (14–30) while maintaining hydrogen permeances in the order of (10⁻⁸–10⁻⁷ mol m⁻²s⁻¹Pa⁻¹) at temperatures as high as 260 °C and do not lose their performance after exposure to 450 °C. The unprecedented thermal stability of these polymer membranes opens the potential for industrial membrane gas separations at elevated temperatures.