The aim of the present study was to find more clues on the nonlinearities in impedance of oxide glasses. The measurements were conducted in three families of glasses in the frequency range of 1 mHz to 10 MHz and the temperature range of -120 degrees C to 150 degrees C. The first family of glasses has a composition of FeO-P2O5-Na2O. Oxide glasses containing iron and alkali are well known for a very low alkali ion mobility and may be considered purely polaron conducting. These glasses show no detectable nonlinearities in the impedance spectra. The second was a family of ionic conducting SiO2-Na2O-Al2O3 glasses. The contents of the third harmonics at certain cases reach high values of about 10% at higher temperatures and extremely low frequency region. This may be caused by blocking properties of electrodes. It has been pointed out that such a nonlinearity may imply errors in interpretation of conventional linear impedance measurements. The third family of glasses contained mixed alkali, no detectable nonlinearities in the impedance spectra was found. (C) 2012 Published by Elsevier B.V.