Polymethylmethacrylate (PMMA) based polymer gel electrolytes containing ammonium hexafluorophosphate (NH4PF6) show conductivity higher than the corresponding liquid electrolytes. Nanodispersed gels prepared by the dispersion of fumed silica with particle size in the nanometer range to polymer gel electrolytes show conductivity of the order of 10(-2) S/cm at 25 degrees C. Two maxima observed at very low concentrations, in the variation of conductivity with the concentration of fumed silica, are related to an increase in free ion concentration due to the dissociation of ion aggregates which is also supported by FTIR results, and the formation of a high conducting interfacial layer between the particles of fumed silica and polymer gel electrolytes respectively. The thermal stability of polymer gel electrolytes shows an improvement with the addition of fumed silica and nanodispersed gels have been found to be thermally stable up to 125 degrees C. The conductivity of nanodispersed gels does not change much over the 20-100 degrees C temperature range and also remains constant with time. (c) 2007 Elsevier B.V. All rights reserved.