Electro-thermal poling of alkali-containing glasses has been known to enhance various physical and chemical properties due to the formation of an alkali ion depletion layer. We have investigated the role of alkali ion migration in depletion layer formation by in situ impedance spectroscopy, poling current measurements and ToF-SIMS on two binary alkali (lithium and sodium) disilicate glasses and two mixed alkali lithium-sodium disilicate glasses. Typically, the depletion layer is formed within a few minutes, reaching a thickness of the order of 100 nm while its impedance continues to increase for the duration of poling. Its electrical conductivity is six or more orders of magnitude lower than that of the bulk glass; by comparison the dielectric constant is lower, approaching the value for silica containing a few percent alkali oxide. Two processes contribute to the formation of a depletion layer: a relatively fast initial process arising from alkali ion migration, followed by a slower process of either electrolysis or gaseous oxygen evolution near the anode. Implications of electro-thermal poling for the mechanism of recently discovered electric field-induced softening of glass are discussed. (C) The Author(s) 2016. Published by ECS. All rights reserved.