The adsorption of As(III) on cobalt and manganese ferrite nanoparticles (NPs) was studied. The ferrite NPs were synthesized using the Massart-assisted microwave hydrothermal treatment. All the NPs exhibited the spinel structure with a formula such as MxFe3- O-x(4), where M = Co or Mn, and x runs from 0.21 to 1.14. The changes in the stoichiometry caused different effects on the physical properties as well on the As(III) adsorption capacity of the NPs. The adsorption data were fitted in very good agreement with the Freundlich model. It was concluded that As(III) was better attracted to ferrimagnetic cobalt ferrite NPs, given that the arsenic removal was significantly higher than that exhibited by superparamagnetic manganese-substituted ferrite NPs.