The synthesis and properties of novel cross-linked composite sulfonated poly (aryl ether ketone sulfone) containing multiple sulfonic side chains as proton exchange membranes were investigated. The multiple side-chain membranes were prepared by direct polycondensation and simple free radical reaction. The cross-linked composite membranes were prepared by solvent coating method. The prepared membranes showed lower water absorption and swelling ratio than that of Nafion 117. The S-Am-2.0/C exhibited the highest proton conductivity of 0.135 S cm(-1) at 80 degrees C, while its swelling ratio was only 3.59% at the same temperature. Much lower methanol permeability coefficients in the range of 3.11 x 10(-8) cm(2)s(-1)-7.25 x 10(-7) cm(2)s(-1) compared to Nafion 117 (2.29 x 10(-8) cm(2)s(-1)) were also observed for the prepared membranes. Moreover, the S-Am-2.0/C exhibited higher peak power density (121.09 mW cm(-2)) than that of main-chain type C-SPAEKS membrane (66.65 mW cm(-2)). The results indicate that the cross-linked composite membranes are good candidate for fuel cells. (C) 2019 Elsevier Ltd. All rights reserved.