The quenching by O-2 of the MLCT excited states of 10 Ru(II)-diimine complexes (RuL(2+)) of the form Ru(bpy)(3-m-z)(bpm)(m)(bpz)(z)(2+) (bpy = 2,2’-bipyridine, bpm = 2,2’-bipyrimidine, bpz = 2,2’-bipyrazine, m and z = 0, 1, 2, 3 and m + z less than or equal to 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(q), have been determined in H2O and D2O and range between 5.1 x 10(8) and 3.4 X 10(9) M(-1) s(-1). The quantum yields (Phi(Delta)) of singlet molecular oxygen, O-2((1) Delta(g)), originating from the energy-transfer quenching of *RuL(2+) by O-2 and determined in D2O by comparison with Phi(Delta) from the excited state of tetrakis(4-sulfonatophenyl)porphine (TPPS4-), are 0.5 for Ru(bpy)(3)(2+) and similar to 1 for all the other complexes. It has been established that for Ru(bpy)(3)(2+) and, presumably, all the other complexes the yield of electron-transfer products, RuL(3+) and O-2(.-), in bulk solution, is negligibly small. The lower value of Phi(Delta) for Ru(bpy)(3)(2+) is attributed to 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.