Abstract: Ferricinium or ferricinium-based compounds are useful as blue redox dyes suitable for monitoring various enzymatic oxidation processes. A process for preparing such dyes is proposed which involves complexing ferrocene or its derivative with a cyclodextrin. The resulting yellow inclusion complex, an intermediate of the redox dye, is then oxidized electrochemically or by a reaction with bilirubin oxidase to form blue redox dyes which exhibit absorption peaks at 620 or 650 nm. Both the reduced form and the oxidized form of the dye are water-soluble.