High-speed spectroelectrochemical methods-a combination of in situ reflection absorption spectroscopy with a 2 ms time resolution with fast-scan cyclic voltammetry or with a potential step method-allowed comprehensive characterization of the reversible oxidation of two-dimensional (2D) J-aggregates of typical cyanine dyes, which were organized on a cysteamine-modified Au(1 1 1). The in situ absorption spectra proved that the 2D J-aggregate could be oxidized reversibly producing well-dispersed dye positive holes that caused distinct electrochromism in the J-band. The coexisting irreversible mode of J-aggregate oxidation was strongly confined to the aggregate domain edges. Data analysis based on the simple Nernst equation allowed a hitherto difficult assessment of the standard potential of the J-aggregate, and suggested that accumulation of dye positive holes caused strongly nonideal redox behavior. The standard potential of the J-aggregate shifted negatively with increasing aggregate size, and its systematic change with the level of J-aggregation had a simple linear correlation with the change of light absorption energy. Independent kinetic information on the interfacial electron transfer rate obtained by the potential step method provided additional support for the results of the redox equilibrium analysis and suggested markedly small reorganization energies of around 0.1 eV for the reversible J-aggregate oxidation. (C) 2003 Elsevier Science B.V. All rights reserved.