The quenching by O[sub 2] of the MLCT excited states of 10 Ru(II)-diimine complexes (RuL[sup 2+]) of the form Ru(bpy)[sub 3[minus]m[minus]z] (bpm)[sub m](bpz)[sub z][sup 2+] (bpy = 2,2'-bipyridine, bpm = 2,2'-bipyrimidine, bpz = 2,2'-bipyrazine, m and z = 0, 1, 2, 3 and m + z [<=] 3) in aqueous solution has been investigated using the techniques of laser flash photolysis and time-resolved and steady-state near-infrared emission spectrophotometry. Values of k[sub q] have been determined in H[sub 2]O and D[sub 2]O and range between 5.1 [times] 10[sup 8] and 3.4 [times] 10[sup 9] M[sup [minus]1] s[sup [minus]1]. The quantum yields ([phi][sub [Delta]]) of singlet molecular oxygen, O[sub 2]([sup 1][Delta][sub g]), originating from the energy-transfer quenching of *RuL[sup 2+] by O[sub 2] and determined in D[sub 2]O by comparison with [phi][sub [Delta]] from the excited state of tetrakis (4-sulfonatophenyl)porphine (TPPS[sup 4[minus]]), are 0.5 for Ru(bpy)[sub 3][sup 2+] and approximately 1 for all the other complexes. It has been established that for Ru-(bpy)[sub 3][sup 2+] and, presumably, all the other complexes the yield of electron-transfer products, RuL[sup 3+] and O[sub 2][sup [center dot][minus]], in bulk solution, is negligibly small. The lower value of [phi][sub [Delta]] for Ru(bpy)[sub 3][sup 2+] is attributed tomore » competitive charge-transfer quenching, followed by efficient back electron transfer within the solvent cage. The values of the rate constants of energy transfer for the six complexes that contain at least one bpz ligand decrease with increasing driving force. 63 refs., 5 figs., 2 tabs.« less