A novel composite membrane of sulfonated graphene oxide (s-GO) and SPEEK with various sulfonated graphene oxide contents was prepared and investigated. The sulfonation of graphene oxide and PEEK induces an increase in the number of sulfonic groups (SO3H), which significantly increases the proton conductivity of sulfonated graphene oxide/SPEEK membrane. The introduction of sulfonated graphene oxide in membranes not only improved proton conductivity, but also positively affected the mechanical properties and the blocking of methanol and water molecules passing through the membrane. Consequently, incorporation of sulfonated graphene oxide into SPEEK membrane has considerably increased the selectivity of the membrane. Therefore, this sulfonated graphene oxide/SPEEK membrane is a good candidate for usage in direct methanol fuel cells. (c) 2012 Elsevier B.V. All rights reserved.