A simple and efficient approach to orange II (AOII) degradation using Co2+-HCO3- system as the catalyst and H2O2 as the oxidant under mild reaction conditions was reported. Complete decolorization of 50 mu M AOII was observed with 5 mu M Co2+ ions and 4 mM H2O2 in 10 mM NaHCO3 aqueous solution after 10 min, which was much faster that that of Fenton reagent with 50 mu M Fe2+ ions. Some intermediate products such as carboxylic acids were detected by electrospray ionization-ion trap mass spectrometry and ion chromatography. In the presence of other pollutants such as methylene blue, sulforhodamine B, and a significant amount of anions including Cl-, CO32-, SO42-, NO3-, HPO42-, and CH3COO-, AOII was still rapidly degraded. The results of photoluminescence probing technology, electron spin resonance spin-trapping technology, and radical trap experiments indicated that the tightly associated hydroxyl radicals with the cobalt complex were the reactive species for AOII degradation.