Synthetic alkaline birnessites with the average Mn oxidation states of 4.02、3.85 and 3.70 are applied to investigate the kinetic characteristics of As(Ⅲ) and Cr(Ⅲ) oxidation. The results show that initial stage of As(Ⅲ) and Cr(Ⅲ) oxidation by Bir-OHs comply with the first order kinetics and then the apparent rate constant (k_obs) decrease with the reaction down to zero due to both the effects of the reverse reaction and the reaction site passivation resulting from the product accumulating on the mineral surface. Nearly no Mn(Ⅱ) release was detected in the initial reaction stage of As(Ⅲ) oxidation, Mn(Ⅱ) were adsorbed to the surface of mineral and then shielded the reaction sites, thus the k_obs decreased sharply. In contrast, for Cr(Ⅲ) oxidation, although Mn(Ⅱ) release lagged behind of Cr(Ⅵ) release in the initial reaction stage, then more and more Mn(Ⅱ) were released to the solution as the reaction went on, the ratios of released Mn(Ⅱ) to Cr(Ⅵ) gradually increased up to the theoretical reaction stoichiometry. The effect of Mn(Ⅱ) shielding the surface reaction sites in the Cr(Ⅲ) oxidation is much less than that in the Cr(Ⅲ) oxidaton, the k_obs decreases much slowly after the initial reaction stage. The higher the Mn AOS, the longer it maintained the first order kinetics in the initial reaction stage. Therefore, alkaline birnessite with higher Mn AOS exhibits the more rapid oxidation reaction towards As(Ⅲ) and Cr(Ⅲ), and the behavior of product release to the solution is a significant factor determining the kinetics of the reaction.