Reaction pathway information and transition states are crucial for understanding adsorption mechanisms of pollutants, such as dimethylarsinic acid (DMA), at the liquid-solid interface. We report a detailed computational analysis of the complexes of DMA on iron (oxyhydr)oxides, including activation energies, transition states, Gibbs free energies of adsorption, Mulliken charges, charge redistribution upon adsorption, and stretching frequencies of As-O bonds for comparison with experimental spectroscopic data. Calculations were performed using density functional theory (DFT) at the B3LYP/6-311+G(d,p) level using both implicit and explicit hydration. For comparison, calculations were also performed for arsenate. Dispersion corrections were included since experimental data showed that DMA forms mostly outer-sphere complexes. Calculated electronic energies indicate that dispersion corrections are important when dealing with outer-sphere complexes, and that there is a high activation barrier of ca. 43 kJ mol-1 to transition from mono- to bidentate DMA complexes. Additionally, extending the modeled iron (oxyhydr)oxides surface to include four Fe centers and analyzing the charge distribution upon adsorption of DMA reveals that electrostatics play a role in the transition from outer-sphere to monodentate complexes. The significance of our results for the overall surface complexation mechanism of DMA and arsenate is discussed.