In the presence of high electrolyte concentration to screen the Coulomb repulsion, the emission of Pt-3(2)(pop)(4)(4-) (where pop = mu-pyrophosphite-P,P') can be quenched by the complexes Co(CN)(5)X3- (where X = I, Br, Cl, N-3). In acidic aqueous solutions the quenching involves the eventual reduction of the cobalt complexes to Co-aq(2+) and gives the oxidative quenching products Pt-2(III)(pop)(4)X-2(4-) (X = I, Br, Cl). For the reaction of Pt-3(2)(pop)(4)(4-) with Co(CN)(5)I3- and Co(CN)(5)Br3- in 0.5 M KCl/pH 2 solution, the product yields per quenching event are 0.34 and 0.21, respectively. Steady-state photolysis using the Co(CN)(5)I3- quencher in 0.5 M KCl medium gave predominantly Pt-2(pop)(4)I-2(4-) as product with only small amounts of Pt-2(pop)(4)ICl4-, pointing to a direct iodine atom-transfer process. In contrast, Co(CN)(5)Br3- quencher gave Pt-2(pop)(4)Cl-2(4-) under the same conditions, possibly indicative of electron transfer. However, study of the mixed-valence platinum dimer intermediates formed by laser flash transient spectroscopy provides conclusive evidence for atom transfer as the dominant primary reactive quenching step for all three halopentacyanocobaltates. The steady-state results for Co(CN)(5)Br3- quencher arose via the interconversion of the monobromo mixed-valence platinum dimer intermediate to its chloro analogue in chloride solutions. Co(CN)(5)N-3(3-) gave a mixture of oxidation products and no intermediate corresponding to electron transfer, indicating a complex interaction with Pt-3(2)(pop)(4)(4-).