A series of novel ionic cross-linking sulfonated poly(ether ether ketone) (SPEER) membranes containing the diazafluorene functional group are synthesized to reduce the swelling ratio and methanol permeability for direct methanol fuel cell (DMFC) applications. The ionic cross-linking is realized by the interaction between sulfonic acid groups and pyridyl in diazafluorene. The prepared membranes exhibit good mechanical properties, adequate thermal stability, good oxidative stability, appropriate water uptake and low swelling ratio. Moreover, the ionic cross-linked membranes exhibit lower methanol permeability in the range between 0.56 x 10(-7) cm(2) s(-1) and 1.8 x 10(-7) cm(2) s(-1), which is lower than Nafion 117, and they exhibit higher selectivity than Nafion 117 at 30 degrees C on the basis of applicable proton conductivity. (C) 2015 Elsevier B.V. All rights reserved.