The reduction of trans-azobenzene. Azb, has been studied in acetonitrile in the absence and presence of added water. The first step of reduction involves formation of the anion radical, Azb(center dot-), and in the second Azb(center dot-) is reduced to Azb(2-) followed by rapid protonation by water to give the monoanion of hydrazobenzene, AzbH(-). This second step of reduction was a candidate for a concerted proton-electron-transfer reaction (CPET) but it turned out instead to be electron transfer followed by proton transfer. From studies of the formal potential of the first step as a function of water concentration, formation constants for the 1:1 (Azb(center dot)(H(2)O)) and 2:1 (Azb(center dot-) (H(2)O)(2)) hydrogen-bonded complexes were evaluated. Proton transfer from water to Azb(center dot-) occurs leading to AzbH(center dot), which in turn is reduced to AzbH-. Kinetic analysis of the voltammetric results indicates that this proton transfer occurs within the 3:1 hydrogen-bonded complex, (Azb(center dot-)(H(2)O)(3)), a species with a very small formation constant, 0.3 M(-1). Two other derivatives, trans-4,4'-dichloroazobenzene and trans-4-dimethylaminoazobenzene were studied briefly and found to behave similarly to Azb. (c) 2009 Elsevier B.V. All rights reserved.