The cyclic voltammogram of chromium (VI) in aqueous acid solution shows a reduction peak at 0.3–0.4V when a platinum electrode is used vs. Na SCE. The reaction is electrochemically irreversible, and, with phosphoric acid in the solution, the voltammogram is different than with any other acid because an insoluble chromium (III) phosphate film is formed on the electrode. This film is insulating and forms in a self‐healing manner. Although it passivates the electrode towards further Cr(VI) reduction, it remains active to protons and oxygen. The film is very tight, and the diffusion of small molecules such as ferrocene across the film is significantly reduced. In aqueous sulfuric acid, the Cr(VI) reduction reaction is diffusion limited with no indications of adsorption on the electrode surface either before, during, or after the reaction.