The Mn-II (HCO3-)-H2O2 (Mn-II-BAP) system shows high reactivity toward oxidation of electron-rich organic substrates; however, the predominant oxidizing species and its formation pathways involved in the Mn-III-BAP system are still under debate. In this study, we used the Mn-II-BAP system to oxidize As(III) in that As(III), Mn2+, and HCO3- are common components in As(III)-contaminated groundwater. Kinetic results show that Mn-II (HCO3-)(n) [including Mn-II(HCO3)(+) and Mn-II(HCO3)(2)] is a key factor in the Mn-II-BAP system to oxidize As(III). Quenching experiments rule out contributions of OH center dot and O-I(2) to As(III) oxidation and reveal that O-2(center dot-) and the oxidizing species generated from O-2(center dot-) play predominant roles in the oxidation of As(III). We further reveal that the MnO2+(HCO3-)(n) intermediate generated in the reaction between Mn-II(HCO3-)(n) and O-2(center dot-), instead of O-2(center dot-), is the predominant oxidizing species. Although CO3 center dot- also contributes to As(III) oxidation, the high reaction rate constant between CO3 center dot- and O-2(center dot-) indicates that CO3 center dot- is not the predominant oxidizing species in the As(III)-Mn-II-BAP system. In addition, the presence of Mn(III) further indicates the important Mn(II)-Mn(III) cycling in the Mn-II-BAP system. We therefore suggest two important roles of Mn-II(HCO3-)(n) in the Mn-II-BAP system: (i) Mn-II (HCO3-)(n) reacts with H2O2 to form the Mn-II(HCO3-)(3) intermediate, followed by a subsequent reaction between Mn-II(HCO3)(3) and H2O2 to produce O-2(center dot-); (ii) Mn-II(HCO3-)(n) can also stabilize O-2(center dot-) with the formation of MnO2+ (HCO3-)(n). MnO2+(HCO3-)(n) is an electrophilic reagent and plays the predominant role in the oxidation of As(III) to As(V).