Copper-catalyzed dismutation has been proposed as an important sink of photoproduced superoxide radical (O-2(-)) in seawater. We have determined the effective rate constants of the reactions of superoxide with Cu(I) and Cu(II) in seawater, using pulse radiolysis and kinetic spectrophotometry. Due to the effects of ionic strength and complexation, the observed rate constants, 1.98(+/-0.05) x 10(9) M-1 s(-1) and 0.663(+/-0.071) x 10(9) M-1 s(-1), for Cu(I) and Cu(II), respectively, are 1 order of magnitude lower than those reported for Cu+ and Cu2+ in low ionic strength media. Measurements of the rate of O-2(-) decay in the presence of catalytic concentrations of dissolved Cu agreed well with predictions based on the independently measured rate constants. Observed steady-state concentrations of Cu(I) in solutions exposed to a constant flux of O-2(-) were also consistent with the measured rate constants. From these results and available data regarding Cu speciation in coastal and open oceans, we calculate that O-2(-) reactions with Cu will be a more important sink of O-2(-) than bimolecular dismutation, even if all organically complexed Cu species are unreactive.