Flavoproteins catalyze many important biological redox reactions. The understanding of the mechanism of their reaction is of primary importance. Because the redox potential (E1) is an equilibrium property of the enzyme, it has an importance of its own. It can also serve as a reference for further studies, and as a convenient basis for the classification of flavoproteins. A previous study, which is most pertinent to our work, indicates that the protein matrix affects both redox properties and catalytic activities of the flavin. Redox data and chemical properties for two different, well characterized flavoenzymes from two different classes, which both have flavin as the redox active compound, indicates that electron transfer occurs by entirely different mechanisms. This stresses the need for a very detailed study of redox potentials of both enzymes and free flavins. Despite the importance of redox properties, E1 and n (number of electrons transferred), for understanding the mechanism, no systematic study of redox potentials of flavoenzymes has been undertaken to date. The aim of the research proposed here is to determine electrochemical properties, primarily E1 and n for selected flavoproteins. We will examine the simplest representatives of the three enzymatic classes which include dehydrogenases, monooxygenases and oxidases. The availability, in the literature, of these fundamental thermodynamic parameters should enable us and others to reach a better understanding of catalytic mechanism of flavoproteins. The spectroelectrochemical method can provide information about E1n and molar absorptivity (epsilon). The method is not limited by the redox potential of a chemical reducing agent because any oxidized mediator dye can be used as the electron transferring agent. In previous attempts to use electrochemistry to determine redox potentials for other enzymes, the anaerobic procedure was not sufficiently rigorous to permit reproducible and reliable measurements for all flavoenzymes. We have developed a spectroelectrochemical cell whose 02 leak rate is twenty-fold lower than that of the most advanced cell to date. Thus, we were able to successfully work at potentials more negative than -0.400 V for up to 12 hours.