Redox reactions of eosin Y (EY) in aqueous solutions in the presence and absence of Zn2+ were studied by spectroelectrochemical measurements combining cyclic voltammetry and in situ monitoring of absorption spectra, employing an optically transparent thin layer electrode (OTTLE). In the absence of Zn2+, the original dianion, EY2-, undergoes one electron reduction to trianion radical that is stable above pH 11, but is protonated and doubly reduced under the lower pH to become highly stabilized against reoxidation. When Zn2+ is present, the reduction occurs as a two electron process coupled with complexation of two Zn2+ ions to bridge eosin Y molecules, resulting in polymeric mixed aggregates that are also greatly stabilized against re-oxidation. This reaction has been identified as the key process in the electrochemical self-assembly of ZnO/eosin Y hybrid thin films in nanowire structures. On the other hand, two electron oxidation of EY2- was found to convert the molecule into lactone form which is strongly stabilized against re-reduction. Despite of such highly complex redox reactions, these processes in overall were found to be reversible both electrochemically and spectroscopically. (C) 2011 Elsevier B.V. All rights reserved.