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Irradiation of Ru( η 6 -mes) 2 2+ (mes is 1,3,5-trimethylbenzene, or mesitylene) in acetonitrile results in the substitution of one mesitylene ring by solvent to yield the half-sandwich complex, Ru( η 6 -mes)(CH 3 CN) 3 2+ . The quantum efficiency of this process exhibits little dependence upon the excitation wavelength or the presence of oxygen, but varies with solvent in the order acetonitrile > water > acetone. Photolysis of Ru( η 6 -mes)(CH 3 CN) 3 2+ in acetonitrile occurs via two competing pathways: substitution of mesitylene by solvent to produce Ru(CH 3 CN) 6 2+ , and exchange of coordinated CH 3 CN with bulk solvent. Quantum efficiencies for both pathways increase with decreasing excitation wavelength and, at each wavelength, CH 3 CN exchange is more efficient than mesitylene substitution. Photochemical mechanisms for these processes are discussed and comparisons with related complexes are presented.