The kinetics of bleaching of the azo dye methyl orange (4-[[(4-dimethylamino)phenyl]-azo]benzenesulfonic acid sodium salt) by hydrogen peroxide both alone and catalysed by [Moo(4)](2)- and [WO4](2-) have been studied at 25 degrees C at pH 10. The effects of adding the sequestrant CIX (diethylenetriaminepentakis-methylenephosphonic acid), free radical and singlet oxygen traps were investigated. It was established that two parallel catalytic pathways were involved: bleaching by peroxomolybdate or peroxotungstate species and oxidation by singlet oxygen, the latter being generated by peroxo species. (c) 2005 Elsevier B.V. All rights reserved.