Abstract This paper is to describe the development of a synthetic strategy for the preparation of phosphonic acid functionalized poly(aryloxyphosphazene) membranes with different substitution degree of phosphonic acid. Synthesized polymers have been characterized by standard spectroscopic techniques; FT-IR, 1 H, 31 P and 19 F NMR and element analysis. Proton conductivities of phosphonic acid substituted polymers have been investigated with impedance spectroscopy at different temperatures. Furthermore, the correlations of the proton conductivity and ion exchange capacity with the substitution degree of phosphonic acid have also been investigated. Thermal properties and water uptake properties of the polymers are also investigated. It is found that the proton conductivity and initial decomposition temperature of the polymers increases up to an appropriate amount of phosphonic acid substitution degree, which can be a useful PEM candidate for fuel cells.