In this work, we have studied the decolouration of 0.10 mM methyl orange in the presence of (1) H2O2, (2) oxone, (3) the combination of Fe2+/H2O2, (4) the combination of Co2+/oxone, (5) the combination of Co2+/H2O2, (6) the combination of Fe2+/oxone. Under the experimental conditions, the data show that H2O2 itself cannot oxidize methyl orange, oxone itself can oxidize methyl orange; all combinations can oxidize methyl orange. The acceleration sequence is: Co2+/oxone > oxone > Fe2+/oxone > Fe2+/H2O2 > Co2+/ H2O2 > H2O2 (no effect). The roles of Fe2+ and Co2+ are catalysts, through recycling-catalysis of M2+ (M = Fe, Co) in the combination of M2+/oxidant (M = Fe, Co; oxidant = H2O2, oxone), a great quantity of HO are generated, active HO can attack methyl orange molecules and accelerate the decolouration of methyl orange. The accelerations from the combinations of oxone and different Co2+-salts (CoSO4, CoCl2, Co(NO3)(2), CoAc2) are almost the same for the decolouration of the azo dye methyl orange (with one azo bond) or congo red (with two azo bonds). (C) 2013 Elsevier B.V. All rights reserved.