Various recast Nafion(R) composite membranes containing ceramic oxide fillers with different surface characteristics (SiO2, SiO2-PWA, AL(2)O(3), ZrO2) have been investigated for application in high temperature direct methanol fuel cells (DMFCs). Cell resistance at 145 degreesC increases as a function of the pH of slurry of the inorganic filler indicating a strong influence of the acid-base characteristics on the electrolyte conductivity. This effect has been attributed to the different water retention capabilities of the various membranes. Fuel cell performance at 145 degreesC, expressed as both maximum power density and current density at 0.5 V cell potential, increases almost linearly as the pH of slurry of the oxide materials decreases. Appropriate selection of the surface properties for the inorganic fillers allows to enhance the proton conductivity and extends the operating temperature range of composite membranes. The influence of fuel cell operating pressure on the humidification properties of these electrolytes at high temperature has been also investigated. (C) 2003 Elsevier B.V. All rights reserved.