In the presence of moderate to high concentrations of electrolytes, the emission of *[Pt-2(pop)(4)](4-) (where pop = mu-pyrophosphite-P,P’) is quenched by the complexes [Co(CN)(5)X](3-) (where X = N-3(-), I-, Br-, Cl-, but not CN-). The salt effects on the emission decay lifetime quenching rate constants between these anionic species have been studied in the presence of MCl, M’Cl-2, or RnNHL4-nCl (where M, M’, and R represent alkali, alkaline earth metals, and alkyl respectively, n = 0-3) and KnX (X = Cl-, Br-, NO3-, SO42-, [Co(CN)(6)](3-), n = 1-3). At 0.5 M cation concentration, second-order quenching rate constants, k(q), are in the "nearly diffusioncontrolled" range, 10(7)-10(9) L mol(-1) s(-1), and k(q) decreases by an order of magnitude across the series of quenchers [Co(CN)(5)I](3-) > [Co(CN)(5)N-3](3-) > [Co(CN)(5)Br](3-) > [Co(CN)(5)Cl](3-). On the basis of a detailed study of [Co(CN)(5)I](3-), the quenching efficiency increases with background electrolyte concentration and the measured rate constants are in good agreement with predictions based on the Debye-Smoluchowski and Debye-Eigen equations for diffusion-controlled formation and dissociation in ionic solution of an encounter pair, together with a rate constant of 1.2 x 10(9) s(-1) for the quenching step. However, the analysis provides further evidence for the Olson-Simonson effect; that is, in the presence of multivalent electrolyte ions, the salt effects are determined by the counterion concentration, here the cation, rather than by the ionic strength. Specific cation effects are observed such that the quenching rate constants increase in the following sequences : Li+ < Na+ < K+ < Cs+; Mg2+ < Ca2+ < Sr2+ < Ba2+; NH4+ < MeNH3+ < Me2NH2+ < Me3NH+; Et3NH+ < Et2NH2+ < EtNH3+; n-PrNH3+ < EtNH3+ < MeNH3+. For the alkali or alkaline-earth cations the large effects seen require participation of the cation in the transition state for the quenching step; the alkylammonium cations are also effective in this role, but the small differences in their efficiencies can be rationalized in terms of their effects on water structure.