Various recast Nafion® composite membranes containing ceramic oxide fillers with different surface characteristics (SiO2, SiO2-PWA, Al2O3, ZrO2) have been investigated for application in high temperature direct methanol fuel cells (DMFCs). Cell resistance at 145 °C 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 °C, 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. An infrared spectroscopic investigation of inorganic fillers employed in composite membranes has been carried out. The surface acidity of the fillers appears to influence the bending and stretching vibrational frequencies of the water physically adsorbed on the filler surface. Inorganic fillers characterised by acidic properties undergo a strong interaction with water and enhance the DMFC performance at high temperature.