Adsorption of methylphosphonic acid (MPA) onto goethite (alpha-FeOOH) was studied as a function of pH and the concentration of MPA in solution. In situ attenuated total reflectance-FTIR (ATR-FTIR) techniques were used to identify the type of surface complexes formed under each of the solution conditions used in the adsorption studies. Additionally, electrophoretic mobility measurements were performed in each of these systems. At low pH and high Gamma, MPA was bound to the surface of goethite predominantly as a monodentate protonated species, while at high pH and low Gamma the prevailing structure was that of a bridging bidentate complex. The change in denticity of the surface MPA under different solution conditions may explain the charge reversals shown in the electrophoretic mobility curves.