In situ observation of absorption spectra of heptyl viologen cation radical (HV+.) was performed by slab optical waveguide (SOWG) spectroscopy combined with the electrochemical techniques. The time and potential dependent adsorption states changes of HV+. molecules, which were adsorbed with monolayer or a few layers on indium-tin-oxide (ITO) electrodes, were characterized in the one-electron reduction of the dication HV2+ to HV+.. Following the monomer deposition, the dimer was adsorbed and coexisted with the monomer on the electrode surface. The broadly observed absorption band ranging from 450 to 650 nm was consisted of at least three kinds of the absorption bands. Especially the absorption band at 550 nm, which was labeled "D" in this study and implied the different aggregation forms from the monomer and dimer of HV+. assigned previously, was found and clearly appeared with time. The results revealed that the adsorption states of HV+. were changed with time, the electrode potential, and the concentration of HV in the monolayer or a few layers deposition. (c) 2006 Published by Elsevier B.V.