An unusual photooxidation of a coordinated 4-mercaptopyridine (SpyH) ligand in the [Ru(Hmctpy)(dmbpy)(kappa S-SpyH)](2+) complex (Hmctpy = 4'-carboxy-2,2';6',2 ''-terpyridine, dmbpy = 4,4'-dimethyl-2,2'-bipyridine) takes place under visible and UV irradiation, in aerated acetonitrile. The [Ru(mctpy)(dmbpy)(kappa S-SO(2)py)] sulfinato product has been characterized by a variety of methods, including X-ray diffraction which supports the presence of the Ru-kappa S-SpyH isomer in the starting complex. The photooxidation of the 4-mercaptopyridine ligand enhances the back-bonding interactions in the complex by means of the strongly acceptor 4-pyridinesulfinato-SO(2)py species, increasing the redox potential of the Ru(III)/Ru(II) couple significantly from 1.23 to 1.62 V. It also led to pronounced changes in the electronic and NMR spectra of the complexes, corroborated by DFT and ZINDO-S calculations. A possible mechanism based on referenced data of photooxidation has been proposed, which involves the formation of a reactive oxygen species and intermediate endoperoxide species, yielding a very stable Ru-sulfinato product. This novel species exhibits stronger luminescence (Phi(f) = 0.004) than the starting complex under UV excitation.