Quantitative photochemical production of [Ru(tpy)(bpy)H](+) (Ru-H+) was achieved by irradiation of [Ru(tpy)(bpy)(DMF)](2+) (Ru-DMF2+; DMF = N,N-dimethylformamide) in a tetrahydrofuran (THF) solution containing excess triethylamine (NEW. The mechanism of the Ru-H+ formation was investigated in detail. A photochemical ligand substitution reaction of Ru-DMF2+ in THF proceeded to give [Ru(tpy)(bpy)(THF)](2+) (Ru-THF2+) with a quantum yield of (7.6 +/- 0.7) x 10(-2). In the presence of NEt3, a similar photochemical ligand substitution reaction also proceeded quickly, but the products were an equilibrium mixture of Ru-THF2+ and [Ru(tpy)(bpy)(NEt3)](2+) (Ru-NEt32+) with a considerable amount of Ru-H+ even in the first stage of the photochemical reaction. The equilibrium constant between Ru-THF2+ and Ru-NEt32+ was determined as 6.9 +/- 2.1. Irradiation to Ru-NEt32+ gave Ru-H+ with a quantum yield of (9.1 +/- 0.5) x 10(-3). An important intermediate, Ru-NEt32+, was isolated, and its properties were investigated in detail.