Mn-substituted iron oxyhydroxide (Mn 0.13Fe 0.87OOH) was prepared by the oxidation of ferrous carbonate precipitated from ferrous sulfate and sodium carbonate solutions. X-ray diffraction analysis led to the conclusion that the sample was basically iron manganese hydroxide with bixbyite structure. The sample exhibited a surface area of 101 m 2 g −1 and a pore volume of 0.35 cm 3 g −1. Batch experiments were conducted to study the adsorption of arsenite and arsenate species onto Mn-substituted iron oxyhydroxide (MIOH) and adsorption equilibrium time was evaluated. The temperature of adsorption was varied from 30 to 60 °C. The maximum uptake of arsenite and arsenate was found to be 4.58 and 5.72 mg g −1, respectively. Zeta potential measurements and FT-IR spectral studies were also conducted to study the nature of adsorption. In both cases, adsorption was best described by Langmuir isotherm and activation energies as calculated from a model-free isoconversional method were found to be on the order of 15–24 and 45–67 kJ mol −1 for arsenate and arsenite, respectively.