The reactivity of inorganic arsenic (As) species at the soil/water interface has been extensively studied. However, there are few studies on the reaction mechanisms of  organoarsenic species at soil interfaces, especially monomethylarsenate (MMA) and dimethylarsenate (DMA). These species are prevalent in the environment,  e.g., they have been extensively used as pesticides.  To understand the fate of methylarsenic in the soil environment, MMA and DMA sorption on aluminum oxide will be investigated  at both the macroscopic scale  (adsorption isotherms, adsorption envelopes, and electrophoretic mobility measurements) and the molecular scale using Fourier transform infrared (FTIR) and  x-ray absorption fine structure (XAFS) spectroscopy.