Application of the NN'-(5-nitro-1,3-phenylene)bisglutaramide (NPG) hydroxylation assay for spectrophotometric determination of the rate of generation of oxidizing species in systems containing copper(II) ions/phosphate buffer/H2O2 in the presence of halide and pseudohalide antioxidants was evaluated. A satisfactory correlation was demonstrated between the kinetics of NPG hydroxylation, determined both spectrophotometrically and chromatographically. Employing the assay, efficiency of various halide and pseudo-halide antioxidants was studied in solutions of pH 7 or 9. Among the halides tested, iodide proved to have the highest antioxidant efficiency. The efficiency of thiocyanate is pH-dependent, exhibiting even pro-oxidant properties at pH 7 at lower concentrations. Various bromides exhibited comparative antioxidant behavior at pH 7 and 9, although the reaction rate constants were much lower at pH 7. A pronounced effect of the cation is evident, with the rates of oxidizing species production affected by the length of alkyl chain on the quaternary ammonium bromide. The use of tetrabutylammonium chloride proved that in the case of tetrabutylammonium cation the influence of anion can be neglected. (c) 2005 Elsevier B.V. All rights reserved.