X‐ray photoelectron spectra of colored WO3films, thermodynamic analysis of galvanic behavior of the tungstic oxide cathode, plus other new lines of evidence, clearly establish the previously uncertain mechanism of the electro‐optic effect in amorphous WO3, viz., a simultaneous injection of electrons and positive ions into a film via an electrochemical process, with the consequent development of a low‐xtungsten bronze. The positive ion, in both the films and in tungsten bronzes, remains fully ionized even in the presence of high densities of injected electrons.