A non-aqueous interfacial polymerization method is presented, in which the conventionally used aqueous phase is replaced by the polar aprotic solvent dimethyl sulfoxide (DMSO) with dissolved potassium hydroxide. This superbase facilitates the deprotonation of a variety of aromatic hydroxyl molecules to highly nucleophilic aryloxide anions. The aryloxides dissolve well in the solution and can react readily with hexachlorocyclotriphosphazene in cyclohexane to form thin films of highly cross-linked cyclomatrix polyphosphazene polymers. The approach widely extends the range of possible chemistries, well beyond the polyamides/polyesters/polyurethanes that are generally associated with conventional (aqueous) interfacial polymerization. The monomers selection and reaction conditions can be tuned to obtain thin films that are suitable for specific applications, ranging from membrane gas separation/blocking at elevated temperatures to organic solvent nanofiltration. This new membrane fabrication platform provides a facile and flexible approach to extend the available materials for challenging industrial separations.