In situ absorption spectra of methylene blue (MB) adsorbed on an indium-tin-oxide electrode were observed by slab optical waveguide (SOWG) spectroscopy. The absorption spectrum of the MB obtained by the SOWG spectroscopy coincided with that by conventional spectroscopy for the solution phase, and the MB existed as dimer at 0 V vs. Ag/AgCl. Upon the electrode potential of cathodic condition, the absorption spectra gradually changed showing the appearance of the monomeric species as well as the highly aggregated species of MB. (C) 2003 Elsevier Science B.V. All rights reserved.