Cystine in aqueous solutions reacts chemically with mercury, forming the surface-bound cysteine mercuric thiolate. When the potential of mercury is made less positive this compound is transformed (at a partially free mercury surface in a currentless process and at an occupied electrode under the passage of current) into cysteine mercurous thiolate. The latter compound is also formed in a reversible interaction of cystine with mercury in the potential range where cystine is adsorbed. At the negative end of this range the electroreduction of cystine occurs by the reduction of cysteine mercurous thiolate. At negatively charged mercury, where cystine is not adsorbed, two electrons are transferred directly to the cystine -S-S- bond in an overall irreversible process. On the positive side the dissolution of mercury into solutions containing cystine starts by anodic formation of slightly soluble mercuric cystinate near the electrode surface.