Nafion is the most widely accepted and commercialized membrane for polymer electrolyte membrane fuel cells owing to its high proton conductivity, mechanical stability and cell performance under humidified conditions at low temperature. However, the performance of the membrane deteriorates rapidly above 80 degrees C. In this work, Laponite XLS, a synthetic grade of peptized Laponite clay, is used as inorganic filler to prepare Nafion-clay nanocomposite membranes using solution mixing technique. The presence of sodium pyrophosphate (Na4P2O7) on the surface of Laponite XLS holds the key towards the improvement in proton conductivity of the nanocomposites compared to the virgin Nafion. Incorporation of acid-activated Laponite to Nafion further improves the mechanical property, thermal stability, water uptake and proton conductivity of the resulting nanocomposite due to the presence of in situ generated H3PO4 on the surface of acid-activated Laponite. The proton conductivity of the nanocomposite membrane containing 3 wt% acid-activated Laponite is found to be 270.2 mS/cm compared to 136.2 mS/cm for the virgin Nafion at 110 degrees C. (C) 2011 Elsevier B.V. All rights reserved.