Ru-based coordination compounds have important applications as photosensitizers and catalysts. [Ru-II(bpy)(2)(bpyNO)](2+) (bpy = 2,2'-bipyridine and bpyNO = 2,2'-bipyridine-N-oxide) was reported to be extremely light-sensitive, but its light-induced transformation pathways have not been analyzed. Here, we elucidated a mechanism of the light-induced transformation of [Ru-II(bpy)(2)(bpyNO)](2+) using UV-vis, EPR, resonance Raman, and NMR spectroscopic techniques. The spectroscopic analysis was augmented with the DFT calculations. We concluded that upon 530-650 nm light excitation, (3)[ Ru-III(bpyNO(-center dot))(bpy)(2)](2+) is formed similarly to the (3)[Ru-III(bpy(-center dot))(bpy)(2)](2+) light-induced state of the well-known photosensitizer [Ru-II(bpy)(3)](2+). An electron localization on the bpyNO ligand was confirmed by obtaining a unique EPR signal of reduced [Ru-II(bpy)(2)(bpyNO(-center dot))](+) (g(xx) = 2.02, g(yy) = 1.99, and g(zz) = 1.87 and N-14 hfs A(xx) = 12 G, A(yy) = 34 G, and A(zz) = 11 G). (3)[Ru-III(bpyNO(-center dot))(bpy)(2)](2+) may evolve via breaking of the Ru-O-N fragment at two different positions resulting in [Ru-IV=O(bpy)(2)(bpy(out))](2+) for breakage at the O-vertical bar-N bond and [Ru-II(H2O)(bpy)(2)(bpyNO(out))](2+) for breakage at the Ru-vertical bar-O bond. These pathways were found to have comparable Delta G. A reduction of [Ru-IV=O(bpy)(2)(bpy(out))](2+) may result in water elimination and formation of [Ru-II(bpy)(3)](2+). The expected intermediates, [Ru-III(bpy)(2)(bpyNO)](3+) and [Ru-III(bpy)(3)](3+), were detected by EPR. In addition, a new signal with g(xx) = 2.38, g(yy) = 2.10, and g(zz) = 1.85 was observed and tentatively assigned to a complex with the dissociated ligand, such as [Ru-III(H2O)(bpy)(2)(bpyNO(out))](3+). The spectroscopic signatures of [Ru-IV=O(bpy)(2)(bpy(out))](2+) were not observed, although DFT analysis and [Ru-II(bpy)(3)](2+) formation suggest this intermediate. Thus, [Ru-II(bpy)(2)(bpyNO)](2+) has potential as a light-induced oxidizer.