The interaction of arsenite (As(III)) and arsenate (As(V)) with magnetite nanoparticles has been studied to evaluate the environmental impact of magnetite nanoparticles and possible beneficial use of these nanoparticles in water treatment. The effects of particle size (20--300 nm) and pH on the sorption/desorption of AS(III) and As(V) on magnetite nanoparticles were examined. On a surface area basis, it was found that large and nanometer scale magnetite particles had similar maximum As(III)/As(V) sorption capacities. However, the sorption of As(III) to magnetite showed the same Langmuir sorption slope for the nanometer scale particles as for the large particles, while the sorption of As(V) to magnetite showed a larger Langmuir sorption slope for the nanometer scale particles than for the large crystals. The particle size effect on sorption is constant over a pH range of 4.8 to 8. Sorption of As(V) was found to be greater at lower pH, while sorption of As(III) was not pH sensitive. The sorption data of both As(III) and As(V) have been modeled with the Diffuse Double Layer model. Stronger desorption hysteresis was observed for both As(III) and As(V) sorption to nanometer scale magnetite particles than that to large magnetite particles, which indicates that the decrease of particle size may increase the affinity of sorption. The experimental results showed that magnetite nanoparticles are potentially a very effective sorbent for arsenic removal in water treatment.