In this work, a combination of constituent materials capable of improving the moisture retention and proton conductivity (PC) was incorporated into sulfonated poly (ether ether ketone) (SPEEK) membranes in order to prepare new, self-humidifying composite membranes (SHMs) for proton exchange membrane fuel cells. The property-improving components were incorporated into the cast SPEEK film in an appropriate microstructural architecture to prepare the SHMs with increased water retention and PC. SHMs were therefore prepared with the inclusion of carboxyl-functionalized graphene (G(c)) and phosphotungstic acid (PWA) with varying proportions into the SPEEK film. The structure of the SPEEK/G(c)/PWA composite membranes was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, thermogravimetric analysis and X-ray photoelectron spectroscopy. The physicochemical properties of the composite membranes, such as ion exchange capacity, water uptake, thermal stability and PC, were investigated. This work provides confirmation that self-humidifying properties are improved at temperature above 60 degrees C through a combinational inclusion of G(c) and PWA within SPEEK and the new self-humidifying membranes have potential for use in medium temperature direct methanol fuel cells. Copyright (c) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.