The electrical properties of composite polymer electrolyte systems composed of 49% poly(methyl methacrylate) (PMMA) grafted natural rubber (MG49) host polymer, ammonium triflate (NH4CF3SO3) salt, and SiO2 as a filler were investigated by impedance spectroscopy technique. Attenuated total reflectance-Fourier transformed infrared (ATR-FTIR) spectra were also measured and indicated the occurrence of polymer–salt–filler interaction. The conductivity for the composite ammonium triflate-doped MG49 polymer was studied as a function of temperature between 303K and 373K. The conductivity was found to exhibit Arrhenius behavior indicating that it is thermally activated. Viscosity measurements were also carried out and showed that the mechanical properties of the composite polymer gel electrolytes (CPGEs) are improved. The highest electrical conductivity of the composite electrolyte system (∼7.55×10−3 Scm−1) was observed for the sample containing 8.0wt.% SiO2. The viscosity for that material was high (∼69cP) compared with the electrolyte containing no SiO2 (∼10cP).