Tin‐doped indium oxide films, prepared by dc magnetron sputtering, have been investigated with respect to their etching behavior in a large number of acids. The etch rate in acids other than the halogen acids is extremely low. The films dissolve at a rate convenient for practical use in concentrated aqueous solutions of halogen acids. Experiments in solutions diluted with solvents with a low dielectric constant show that the undissociated halogen acid is the active agent for the etching process. A mechanism is proposed to explain the observed results. Addition of or to solutions increases the etch rate substantially. Voltammograms and etching experiments under potentiostatic control show that this is a nonelectrochemical effect.