This paper reports the development and characterization of phosphonated poly(arylene ether sulfone) polymer electrolytes for direct methanol fuel cells. The synthesis of phosphonated polysulfone was performed by a post-phosphonation method via chloromethylation of the polysulfone backbone followed by phosphonation utilizing the Michaels-Arbuzov reaction. High degree of phosphonation up to 150% was achieved without crosslinking side reactions. The obtained membranes/polymers in the ester form were hydrolyzed to the corresponding phosphonic acid by refluxing in aqueous hydrochloric acid. The modified polymers were characterized by nuclear magnetic resonance, infrared spectroscopy, ion exchange capacity, differential scanning calorimetry and thermal gravity analysis. The high level of phosphonic acid content 150% led to high water uptake level 52 wt% which is necessary to reach high proton conductivity values. The synthesized membranes with the highest phosphonic acid content 150% reached 12 mS/cm at 100 degrees C under fully hydrated conditions and showed low methanol crossover (9.12 x 10(-8) cm(2)/S) compared to Nafion 117 membranes. Also, membranes with 150% phosphonic acid content exhibit high thermal stability up to 252 degrees C under air which entitle them as future candidates for proton exchange membrane fuel cells PEMFCs. (C) 2010 Elsevier B.V. All rights reserved.