Previous studies have shown that the oxidation of Fe(II) by molecular oxygen can lead to the co-oxidation of As(III) at neutral pH. However, the mechanism of As(III) oxidation in the presence of Fe(II) with respect to the interaction between As(III) and Fe(II) is still unclear. In this work, we examined the oxidation of As(Ill) in the presence of Fe(II) in air-saturated water, which is affected by pH, co-existing phosphate, and scavengers (tert-butanol, superoxide dismutase, catalase and dimethylsulfoxide). Results confirm that the formation of the Fe(II)-As(III) complex (formation constant, K-Fe(II)(-As(III))=10(3.86) M-1) plays an extremely important role in the initial stage of As(III) oxidation at pH 7.25. The oxidation of Fe(II)-As(III) promotes the production of H2O2 and colloidal ferric hydroxide. H2O2 reacts with the Fe(II)-As(III) complex through oxidizing Fe(II) to Fe(IV), which then causes the partial oxidation (ca. 50%) of As(III). The other part of As(III) oxidation by H2O2 occurs, in the form of Fe(III)-As(III) complex, through direct electron transfer from As(III) to H2O2 but not through Fe(IV). This work provides new mechanistic insight into arsenic and iron redox chemistry in the environment and furthers our understanding of Fenton reactions at neutral pH. (C) 2018 Elsevier Ltd. All rights reserved.