In this study, the sulfonic acid groups' adequately functionalized graphene oxide (FGO) was prepared using a temperate and controllable method. The morphological changes of the FGO and sulfonated polyarylene ether sulfone (SPES) composite membranes were investigated in detail by using X-ray scattering (SAXS), atom force microscopy (AFM), transmission electron microscopy (TEM), scanning with an electron microscope (SEM), and so on. The SAXS showed an increscent d-spacing, and reflected the enlargement of the ionic clusters, which was caused by the addition of sulfonated groups into the composite membranes. The AFM and TEM showed phase separation phenomenon. The phase separations led to the formation of more continuous and larger channels for proton conducting. Also, the SEM pictures intuitively showed the inner-structures of the membranes. When a small amount of the FGO was blended, the structure of the composite membrane was compact. The compact structures resulted in the improvement of the mechanical properties and methanol resistance. However, when more of the FGO was blended, the structures became loose and caused damage to the mechanical properties. The FGO can effectively hinder the methanol passing through and enhance the water retention by its special sheeted structure. The properties of the composite membrane with a 5% blended FGO was found to be superior to that of Nafion117 and other samples. It was determined that the changed properties of the composite membranes must have been derived from the different microstructures. The relationships between the properties and the morphologies of-the samples were established in detail. Therefore, this study provides helpful information regarding the design of excellent proton exchange membranes. (C) Copyright 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.