The cobalt atom of cobalamins is reduced by cysteamine and other thiols at alkaline pH. The reduction is avoided when the solution of cobalamin is passed through a chelating resin. The addition of a trace of Cu⁺⁺ ions to the solution of cobalamin passed through the chelating resin regenerates the reduction of cobalt by the thiols. This result indicates that the reduction of cobalamins by thiols should be mediated by a trace of metal present as an impurity in the cobalamin.