This study elucidates the decolorization of C.I. Reactive Red 2 (RR2) by homogeneous catalytic ozonation. The effects of pH and catalyst dosage were evaluated in O-3/Mn(II), O-3/Fe(II), O-3/Fe(III), O-3/Zn(III), O-3/Co(II) and O-3/Ni(II) systems. In O-3/Mn(II), O-3/Fe(II) and O-3/Fe(III) systems, increasing the catalyst concentration increased the rate of RR2 decolorization; however, further increasing the catalyst concentration caused no further significant increase. When 0.6 mM catalyst was added, the decolorization rates Of O-3/Mn(II), O-3/Fe(II), O-3/FC(III), O-3/Zn(II), O-3/CO(II) and O-3/Ni(II) systems at pH 2 were 3.295, 1.299, 1.278, 1.015, 0.843 and 0.822 min(-1), respectively. Under all of the experimental conditions, the decolorization efficiency of catalytic ozonation exceeded that of ozonation alone. The decolorization rate markedly exceeds the TOC removal rate in all tested systems. The effect of the radical scavenger on the catalytic ozonation processes suggests that the decolorization reaction in catalytic ozonation systems proceeds by mainly radical-type mechanisms, except in the O-3/Mn(II) system. (c) 2007 Elsevier B.V All rights reserved.