Little work has been conducted on the adsorption of arsenic to the mixed iron [Fe(II)/(III)] oxide magnetite and the effect that environmental parameters, such as pH, ionic strength, and temperature, have on adsorption. Magnetite nanoparticles are unique because of their affinity for both arsenate and arsenite and increased adsorption capacity from their bulk counterparts. This article shows the effect of various magnetite nanoparticle concentrations on arsenic adsorption kinetics. The adsorption data show the ability of the magnetite nanoparticles to remove arsenate and arsenite from solution in both synthetic and natural waters, and the data fit a first-order rate equation. Because of the increased surface area of these particles, less than 1 g/L of magnetite nanoparticles was needed. The results suggest that arsenic adsorption to the nanoparticles was not significantly affected by the pH, ionic strength and temperature in the ranges tested, which are typical of most potable water sources.