Atmospheric Chemistry / Chemical Kinetics / Pulse Radiolysis / Thansition Metals IonsThe catalytic decay of 0; radicals due to Cu+ and Cu2+ ions in 0, saturated solutions of low ionic strength was studied by pulse radiolysis and computer simulation of the entire reaction mechanism. Phosphate was used as buffer and formate was used to convert OH radicals into 0;. At pH = 6, the very low solubility products of Cu2+ with OH-and PO:-limited the copper concentration to the order of wM. In experiments with [Cu*+]>[O,], the rate constant k(Cu2+ +OF) = (8.0rt0.5).109M-'s-1 was determined. In systems with[O?] * [CuItot, steady state for the copper ions is attained with apparent first-order kinetics, which correlates k(Cu2+ +OF) with k(Cu+ +OF). The 0, decay rate decreased with increasing pH due to Cu2+ complexation with HPOZ-. Correspondingly the pK(CuHP04) was found to be -4.Ok0.2 under the assumption that k(CuHP04+O;)S0.01 *k(Cu2+ +O;). With this value, k(Cu+ +O;) could be determined as (9.4k0.8).lo9 M -' s-' by an iterative procedure. The consequences for the chemistry of atmospheric waters are discussed.