In situ observation of absorption spectra of the heptylviologen cation radical (HV+) was performed by slab optical waveguide (SOWG) spectroscopy utilizing indium-tin-oxide (ITO) electrodes. On application of potential sweep and potential step techniques from -300 to -600 mV (vs. Ag vertical bar AgCl), we observed the initial adsorption of HV+ on the ITO electrode. Two specific absorption bands were found at around 380 and 400 nm, which indicated that HV+ existed as both monomer and dimer on ITO with a monolayer or a few deposition layers. The change in the intensity and the peak position of the absorption spectra from adsorption species of HV+ was correlated with the electrode potential. These two absorption bands became incorporated in one broad band after a 30 s interval at -600 mV. (c) 2005 Published by Elsevier B.V.