An effective procedure for selective reduction of O₂ to H₂O₂ exploring the use of hydrogen sulfide, an obnoxious industrial pollutant as reductant is reported herein. The reduction of [Ru^III(EDTA)pz]⁻ (EDTA⁴⁻ = ethylenediaminetetraacetate; pz = pyrazine) by hydrogen sulfide resulting in the formation of a red [Ru^II(EDTA)pz]²⁻ complex (λ_max = 462 nm) has been studied spectrophotometrically and kinetically using both rapid scan and stopped-flow techniques. The time course of the reaction was followed as a function of [HS⁻]_i, pH (5.5–8.5), and temperature. Alkali metal ions were found to have a positive influence (K⁺ > Na⁺ > Li⁺) on the reaction rate. Kinetic data and activation parameters are interpreted in terms of a mechanism (admittedly speculative) involving outer-sphere electron transfer between the reaction partners. Reaction of the red [Ru^II(EDTA)pz]²⁻ complex with molecular oxygen regenerates the [Ru^III(EDTA)pz]⁻ species in the reacting system along with the formation of H₂O₂, a partially reduced product of dioxygen (O₂) reduction. A detailed reaction mechanism in agreement with the spectral and kinetic data is presented.