An in situ spectroelectrochemical method has been successfully used to study the electrocatalytic oxidation of guanine on an anisomerous ruthenated porphyrin [Ru(bpy)(2)(MPyTMPP)Cl](+) (bpy = 2,2'-bipyridine and MPyTMPP = 5-(4-pyridyl)-10,15,20-tris(4-metylphenyl)porphyrin) modified indium-tin oxide (ITO) electrode. The [Ru(bpy)(2)(MPyTMPP)Cl](+)/ITO electrode shows a pair of well-defined redox peaks at the formal potential of 0.79 V (vs. Ag-AgCl/0.05 M NaCl) and an intense emission peak at 654 nm. The evidences from photo-induced potentiometric measurement and electrochemically induced emission spectra under the excitation of visible light suggest the formation of Ru(III)-based species, which quenches the luminescence of Ru(II)-based excited states by an analogous Stern-Volmer model. The [Ru(bpy)(2)(MPyTMPP)Cl](+)/ITO electrode exhibits a good linear electrocatalytic response for guanine oxidation between 0.05 and 0.4 mM. The electrocatalytic oxidation of guanine is found to fully restore the characteristic spectral response weakened by Ru(III)-based species. Taken together, the results from this study provide a powerful basis for better understanding the electrocatalytic and photo-induced oxidation of analytes by dye-redox mediators. (C) 2013 Elsevier B.V. All rights reserved.