Partially sulfonated poly(2,6-di-methyl-1,4-phenylene oxide) (sulfonated PPO) with various degrees of sulfonation were prepared. The solutions were mixed with organically modified montmorillonite (MMT) to prepare membranes by solvent casting. By increasing the sulfonation degree up to 40% for membranes without MMT, ion exchange capacity, water uptake and proton conductivity reached 2.59 mequiv. g(-1), 21 % and 0.0182 S cm(-1), respectively. The Fourier transfer infrared (FTIR) analysis of sulfonated membranes revealed absorption bands at 1060 and 1100-1300 cm(-1) for sulfur-oxygen S=O bonds. X-ray diffraction analysis showed the exfoliated structure of clay in polymeric matrices. A sulfonated PPO/MMT membrane with 27% sulfonation and 2.0 wt% MMT loading showed a membrane selectivity of approximately 63,500 compared to 40,500 for Naflon (R) 117, and also a higher power density (125 mW cm(-2)) than Nafion (R) 117 (108 mWcm(-2)) for single cell DMFC in a 5 M methanol feed. (C) 2008 Elsevier B.V. All rights reserved.