The mechanism of oxidation of As(III) in alkaline solution by air with promotion effect of KMnO4 was studied. The experimental results indicated that the superstoichiometric oxidation of As(III) by KMnO4 could be attributed to the catalytic effect of reductive product of KMnO4. The XRD and XPS results demonstrated that the catalyst was nascent MnO2 rich in potassium. The results also showed that the mole ratio of Mn/As and the initial pH had significant effects on the oxidation of As(III). The time for the oxidation by air was less than 2h with the mole ratio of Mn/As less than 1/10.5 and the initial pH higher than 13. The kinetics of the catalytic oxidation of arsenic was interpreted using the pseudo first order reaction, and the apparent active energy was about 15.01 KJ/mol. The study suggested that the initial oxidation was firstly dominated by the direct oxidation of KMnO4 followed by the catalytic oxidation with the nascent MnO2. (C) 2014 Elsevier B.V. All rights reserved.