Photolysis of Ru(bpy)(2)(en)(2+) and Ru(bpy)(2)(tn)(2+), where bpy = 2,2’-bipyridine, en = ethylenediamine, and tn 1,3-propylenediamine, was studied in acetonitrile using on-line electrospray mass spectrometry (ES-MS). These complexes are known to undergo a four-electron oxidation photochemically, giving the alpha,alpha’-diimine complexes. The monoimine complexes involved in this stepwise process were detectable after photoirradiation (lambda >420 nm). Also, new ligand-oxidized complexes Ru(bpy)(2)(en+14)(2+) and Ru(bpy)(2)(tn+14)(2+) were observed together with photosubstitution products such as Ru(bpy)(2)(AN)(2)(2+) and Ru(bpy)(2)(AN)(2)X(+) (AN = acetonitrile). The notation (en+14) and (tn+14) represents loss of two hydrogen atoms and addition of an oxygen atom to the en and tn ligands. Photosubstitution intermediates with the monodentate diamine, Ru(bpy)(2)(tn)(AN)(2+) and Ru(bpy)(2)(tn)(AN)X(+), were detected in the ES mass spectrum of the tn complex but not in that of the en complex. Other photosubstituted intermediates with the monodentate (en + 14) and (tn + 14) ligands were detected by on-line mass analysis. The electrospray technique combined with use of a flow-through photoreaction cell was shown to be a useful tool for studying photolysis of inorganic metal complexes.