To examine some effects of ozonizing seawater components, residual oxidant levels were measured amperometrically in buffered solutions (pH 2.8, 7.0, or 10.4) treated with an O2-O3gas stream at room temperature. The results may be interpreted in terms of competition for O3by Br−; (which preserves the measurable oxidizing capacity of the solution through formation of active Br+species) and OH−(which catalytically destroys the measurable oxidizing capacity by formation of analytically inert O2; and H2O2).