The effect of nanosized silica when incorporated in polymer electrolytes is analyzed by means of Fourier transform infrared (FTIR) spectroscopy, conductivity and thermal properties. Nanocomposite polymer electrolytes are synthesized by the dispersion of nanosized silica (SiO2), up to 10wt.% maximum, into a matrix formed by poly(methyl methacrylate) (PMMA) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The highest conductivity is 2.44×10−6 Scm−1 at room temperature, with 4wt.% of silica added. The FTIR spectra show evidence of complexation between PMMA, LiTFSI and SiO2. The addition of silica to the polymer electrolytes also improves the thermal stability and the ability to retain conductivity over time.