Arsenate and arsenite adsorptions were investigated on a soil. The maximum adsorption of arsenite estimated from the Langmuir adsorption isotherm was somewhat higher than that of arsenate, although in the low concentration range the arsenate adsorption was greater than that of arsenite. Proton consumption was not found for any of the arsenite adsorptions, denying the occurrence of ligand exchange reactions together with proton consumption in the adsorption. The adsorption of arsenate by the soil was constantly about 100% at pH<7.0, while the adsorption decreased with the increasing pH at pH>7.0. The arsenite adsorption by the soil increased with the increasing pH until the pKa value. The results of the desorption experiments showed that the adsorbed arsenate was fixed strongly by soil particles and that the adsorption mechanism of arsenite to the soil particles may be comprised of comparatively weak adsorptions and strong sorptions.