Arsenic toxicity, mobility, and bioavailability in soil-water systems are highly dependent on its oxidation states and chemical species. In this study, both indirect (FTIR) and direct (X-ray Absorption Near Edge Structure (XANES) spectroscopic techniques were applied to investigate the adsorption and oxidation of As(III) on the surface of goethite. The results indicate that at low pH, the As(V)/As(III) ratio in the solid phase is higher than in the solution phase. As(III) adsorbed on goethite under air dry conditions was not stable. After 20 days, more than 20% of adsorbed As(III) was oxidized to As(V). Birnessite was an active oxidant of As(III), both in solutions and on the goethite surface. This study suggests that the adsorption-oxidation system composed of goethite and birnessite may be significant in decreasing arsenic toxicity in terrestrial environments.