The photochemical and luminescence behavior of the orthometalated Pt(thpy)/sub 2/ complex, where thpy/sup -/ is the ortho-C-deprotonated form of 2-(2-thienyl)pyridine, has been studied under a variety of experimental conditions, and the mechanisms of the excited-state reactions and of the subsequent thermal processes are discussed. In degassed CH/sub 3/CN the complex is not photosensitive and exhibits luminescence from the lowest triplet metal-to-ligand charge-transfer (/sup 3/MLCT) excited state with phi/sub em/ = 0.26 and tau = 2.4 ..mu..s. In CH/sub 2/Cl/sub 2/, CHCl/sub 3/, or CH/sub 3/CN/CH/sub 2/Cl/sub 2/ solvents, the complex maintains its luminescent properties and undergoes a photooxidative addition reaction with formation of Pt(thpy)/sub 2/(Cl)(R) (R = CH/sub 2/Cl or CHCl/sub 2/) as the sole observed product. In the mixed solvent, the quantum yield of the photoreaction increases with increasing CH/sub 2/Cl/sub 2/ concentration. In neat CH/sub 2/Cl/sub 2/, the quantum yield of the photoreaction is 0.30 and 0.10 for 313- and 430-nm excitation, respectively. In CH/sub 2/Cl/sub 2/, complete quenching of the luminescent /sup 3/MLCT excited state by anthracene via an energy-transfer mechanism is accompanied by only partial quenching of the photoreaction. By contrast, oxygen is a better quencher for the photoreaction than for the luminescence emission. In bothmore » cases the fraction of quenched reaction depends on the excitation wavelength.« less