A method for the synthesis of phenylphosphonic acid functionalized poly[aryloxyphosphazenes] is described. Diphenyl chlorophosphate was used as the phosphonating agent and was allowedto react with lithio-functionalized aryloxyphosphazenes, followed by treatment with aqueous sodiumhydroxide. Subsequent acidification with aqueous hydrochloric acid yielded phenylphosphonic acidfunctionalized poly[aryloxyphosphazenes]. These polymers are candidates for use as proton-conductingmembranes in fuel cells.