A simple cobalt(II)-monoethanolamine (MEA) complex was used for orange II degradation with hydrogen peroxide as an oxidant under mild conditions. Fast and complete decolorization was observed in an aqueous solution containing 10 mu M Co2+ ions, 1 mM MEA and 6.4 mM H2O2 at room temperature. Several intermediate products were detected by electrospray ionization mass spectrometry. When using other transition metals, or replacing MEA by other chelating agents, the system was much less efficient. The Co2+-MEA complex was pH-dependent, but it exhibited high catalytic activity in the pH range 7-13. It was suggested from electron spin resonance spin-trapping technologies and radical scavenging measurements that the produced hydroxyl radicals were to be responsible for the dye decolorization. This study provided us a simple, effective and economical system ideal for the treatment of toxic and nonbiodegradable azo dye. (C) 2012 Elsevier B.V. All rights reserved.