High temperature proton exchange membranes based on Nation were prepared by incorporating the polymer with ionic liquid cation 1-butyl-3-methylimidazolium (BMIm) and doping with phosphoric acid (PA). We found that using the hydroxide form rather than the chloride form of BMIm incorporated more readily the BMIm cation into Nafion film. A mole ratio of about 2 of BMIm cation to Nation repeat unit, i.e., lambda(BMIm/Nafion), was reached with the hydroxide form BMIm. The incorporated BMIm cation enhanced the doping of phosphoric acid into Nation. A proton conductivity of 10.9 mS cm(-1) and a tensile stress at break of 5.3 MPa were achieved, respectively, with a composite membrane of Nafion/2.3BMIm/5.2PA in molar ratio at 160 degrees C without humidification. (C) 2011 Elsevier Ltd. All rights reserved.