The sulfonated poly(ether ether ketone sulfone) (SPEEKS)/heteropolyacid (HPA) composite membranes with different HPA content in SPEEKS copolymers matrix with different degree of sulfonation (DS) were investigated for high temperature proton exchange membrane fuel cells. Composite membranes were characterized by Fourier transfer infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). FTIR band shifts suggested that the sulfonic acid groups on the copolymer backbone strongly interact with HPA particles. SEM pictures showed that the HPA particles were uniformly distributed throughout the SPEEKS membranes matrix and particle sizes decreased with the increment of copolymers' DS. The holes were not found in SPEEKS-4/HPA30 (consisting of 70% SPEEKS copolymers with DS = 0.8 and 30% HPA) composite membrane after composite membranes were treated with boiling water for 24 h. Thermal stabilities of the composite membranes were better than those of pure sulfonated copolymers membranes. Although the composite membranes possessed lower water uptake, it exhibited higher proton conductivity for SPEEKS-4/HPA30 especially at high temperature (above 100 degrees C). Its proton conductivity linearly increased from 0.068 S/cm at 25 degrees C to 0.095 S/cm at 120 degrees C, which was higher than 0.06 S/cm of Nafion 117. In contrast, proton conductivity of pure SPEEKS-4 membrane only increased from 0.062 S/cm at 25 degrees C to 0.078 S/cm at 80 degrees C. At 120 degrees C, proton conductivity decreased to poor 0.073 S/cm. The result indicated that composite membranes exhibited high proton conductivity at high temperature. (c) 2006 Wiley Periodicals, Inc.