The redox process between NADH in an aqueous solution, W, and chloranil, CQ, or toluquinone, TQ, in 1,2-dichloroethane, DCE, at the W/DCE was investigated as the function of potential differences applied between W and DCE, E-W/DCE TWO different redox reactions were found to occur depending on the potential difference at the W/DCE interface as well as the kind of quinone, Q. The reaction products between NADH and CQ were NAD(+) and CQ(.-) when the electrolysis had been carried out by applying a constant E-W/DCE in one potential range, Range A, and were NAD+ in W and QH(2) in DCE at E-W/DCE in another potential range, Range B. It was also demonstrated that the redox reaction could be controlled by selecting the ion transfer reaction occurring simultaneously with the redox reaction at the interface. (C) 1998 Elsevier Science Ltd. All rights reserved.