A series of polyphosphazene-based copolymers containing alkylsulfonated side chains were prepared and made into the corresponding membranes by the reaction with 2,6-bis(hydroxymethyl)-4-methylphenol (BHMP) as the cross-linking reagent. All the cross-linked membranes showed lower water uptake and dimensional swelling. They displayed significant methanol resistance ranging from 1.35 to 7.18 x 10(-7) cm(2)/s and high oxidative stabilities. The individual membrane PFMPP1-R-PFSPP9 showed good proton conductivity (0.14 S/cm) under fully hydrated conditions at 80 degrees C. (C) 2015 Elsevier B.V. All rights reserved.