In situ electrochemical-absorption spectra of meso-tetrakis (4-N-Methylpyridyl) porphine p-Toluenesulfonate (TMPyP) adsorbed on the surface of an ITO (indium-tin-oxide) electrode was observed by simultaneous measurements using cyclic voltammetry (CV) and internal reflection spectroscopy. The CV experiment was performed using ITO coated on glass as the working electrode, platinum wire as the counter electrode and silver/silver chloride (Ag/AgCl) electrode as the reference electrode. It was found that, in 1 M HCl and pH=2 Britton-Robinson solutions, TMPyP were adsorbed on ITO as P(0)H(4)(2+) state and P(0)H(2) state, respectively. Moreover, when the potential was applied on the ITO electrode, the redox reactions of TMPyP in 1 M HCl were also different from those in pH=2 Britton-Robinson solutions. The adsorption rate, desorption rate and equilibrium constants as well as the adsorption free energy of TMPyP were determined by fitting adsorption process between TMPyP and ITO using Langmuirian adsorption model. The effect of pH on the internal reflection spectra was also explored, in which it was shown that the increase of pH lead to the enhancement of the intensity of the internal reflection spectra and the blue shift of the peak position. With the increase of ionic strength of TMPyP solution, the intensity of the internal reflection spectra decreased. (C) 2008 Elsevier B.V. All rights reserved.