We have prepared and characterized [Ru(bpy)(2)(OS)](+) (bpy = 2,2 '-bipyridine; OS = 2-methylthiobenzoate) and the chelating sulfoxide S-[Ru(bpy)(2)(OSO)](+) (OSO = methylsulfonylbenzoate) by H-1 NMR (1-D and 2-D COSY), IR, UV-visible spectroscopy, electrochemistry, and X-ray crystallography. Many of the metrical and crystal parameters are nearly identical between the two structures. The sulfoxide is produced from m-cpba oxidation of the thioether. Photolysis of S-[Ru(bpy)(2)(OSO)](+) results in a dramatic shift in the metal-to-ligand charge-transfer ((MLCT)-M-3) transition from 396 to 496 nm, with a new higher-energy (MLCT)-M-3 transition appearing at 355 nm. Concomitant with this change, the Ru3+/2+ reduction potential shifts from 1.25 V vs Ag/AgCl to 0.9 V vs Ag/AgCl. These changes are ascribed to phototriggered excited-state isomerization of the sulfoxide from S- to O-bonded. Examination of the H-1 NMR spectra in a CD3OD solvent before, during, and after irradiation shows the presence of two O-bonded complexes that revert to the structurally characterized S- bonded ground state. This represents the first report of isomerization of a chelating sulfoxide in a photochromic Ru complex.