The oxidation of arsenite (As^III) by manganese (Mn) oxides is an important process in the natural cycling of arsenic (As) in the environment. Previous studies have demonstrated rapid oxidation of As^III by Mn oxides which produces the less toxic and less mobile arsenate (As^V) that can be sorbed on mineral surfaces. Under natural conditions, the presence of other ions (i.e., Fe^II) can influence the behavior of As^III on Mn oxides. However, very few studies have focused on the effects of Fe^II on the redox and sorption processes. In this study, As^III oxidation by hydrous manganese oxide (HMO) in the presence and absence of dissolved Fe^II was investigated using stirred-flow experiments. Chemically synthetic HMO was reacted with three influent solutions, containing the same As^III concentrations but different Fe^II concentrations, at pH 4 for 24 hours. The results show an initially rapid As^III oxidation by HMO, which is followed by an appreciably slow reaction after 4 hours. In the presence of Fe^II, As^III oxidation is slightly decreased by 10%. This is due to Fe^II reacting with the HMO surface, decreasing HMO reactivity. Evidence for this was an increase in dissolved Mn^II concentrations as well as a decrease in dissolved Fe^II concentrations in the effluent solutions when Fe^II was present. However, when As^III is oxidized to As^V, the retention of As^V increases greatly in the presence of Fe^II and also increases with increasing Fe^II influent concentrations. This could be attributed to the formation of Fe^III hydroxides on the HMO surface after Fe^II is oxidized, which could provide additional sorption sites for As^V. This study suggests that the oxidative precipitation of Fe^III compounds on HMO surface may play a very important role in As^III oxidation and As^V sequestration. Molecular scale studies are in progress to address the reaction mechanisms.