The surface properties of inorganic fillers employed in composite membranes have been investigated by X-ray photoelectron spectroscopy (XPS), BET surface area and acid-base characterisations. The adsorption density of hydroxyl-hydrogen ions and the pH of slurry varied with the amount and strength of acid-base surface functionalities. A shift towards higher binding energies was observed for the O 1s photoelectron peak as a function of the increase of the filler acidic behaviour. The conductivity of the membranes and the maximum power density at 145 degreesC for direct methanol fuel cells (DMFCs) increased with the filler surface acidity. (C) 2003 Elsevier B.V. All rights reserved.