The etching of indium-tin oxide (ITO) in acidic solution has been investigated electro-chemically via conductivity change, cyclic voltammetry and in situ UV-visible spectroscopy. It is seen that the ITO has a new color and lower conductivity when reduced. Proton insertion is faster than proton extraction, with JOURNAL/jmsme/04.02/00009910-199910020-00004/ENTITY_OV0430/v/2017-11-08T150812Z/r/image-png = 7.0 × 10−11 cm2 s−1 and 2.4 × 10−11 cm2 s−1 respectively. Spectroscopic and kinetic results are discussed in terms of a possible mixed-valent solid-state material comprising In(I)–In(III) oxides, although it is concluded that such a system does not, in fact, exist. It is shown that the chemical reactivity of reduced ITO is more-or-less the same as the oxidized material, but the observed changes result from a decreased mechanical strength causing HxITO to disintegrate faster than ITO. Implications are discussed for ITO-based devices that contain traces of aqueous acid.