The quenching of the Ru(bipy)32+ phosphorescence by Co(NH3)5X3+ (X = Br-, Cl-, H2O and NH3) and by Ru(NH3)5X3+ (X = Cl- and NH3) complexes has been studied by a spectrofluorimetric technique. Stern-Volmer constants were determined and the kinetics of the quenching reactions investigated in 0.5 M sulfuric acid at 25°. Some measurements were also performed in 50% 2-propanol. The Co (NH3)5X3+ quenched rates increase in the order NH3 < H2O < Cl- < Br- with the latter reaction being diffusion controlled. In the absence of 2-propanol the ratio k2/kQ, where k2 is the second-order rate constant for the formation of Ru(bipy)33+ and kQ is the second-order quenching rate constant, was close to unity for all of the cobalt(III) complexes studied. This ratio was somewhat lower for Co(NH3)5Cl2+ in 50% 2-propanol. The Ru-(NH3)5X3+ complexes quenched much more efficiently than the corresponding Co(NH3)5X3+ complexes; however, in contrast to the cobalt(III) reactions, the ruthenium(III) reactions were not accompanied by the net oxidation of Ru(bipy)32+. The results are discussed in terms of an electron transfer quenching mechanism, and a value of 0.9 V is estimated for the oxidation potential of 3Ru(bipy)32+.