Magnetite (Fe3O4) nanoparticles (MNs) are largely known as strong sorbents for inorganic ions such as divalent transition metals (e.g. Co2+). Therefore, MNs play an important role on the behavior and fate of trace contaminants, and are commonly used in contaminated water treatment technologies. In addition, surface modification of MNs using Co2+ affects MNs magnetic properties, which leads to a broad range of high technology applications (e.g., catalysis, medicine and electronics). However, the mechanisms involved between fully stoichiometric magnetite (i.e., with Fe(II)/Fe(III) = 0.5) and transition metals in aqueous solutions are still poorly understood. Adsorption of Co onto stoichiometric MNs (~10 nm-sized) was studied at pH = 8 under inert atmosphere to ensure no evolution of MNs stoichiometry. The Co adsorption isotherm was found non-linear over the 5 orders of magnitude in aqueous [Co] investigated. Adsorption modeling, soft X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) at the Co and Fe L2,3-edges evidenced three types of surface species, which could be attributed to (i) surface complexed or incorporated Co2+ with a ferrimagnetic behavior at low loadings, (ii) magnetically-silent small Co polymers at intermediate loadings and (iii) precipitation of antiferromagnetic Co(OH)2(s)-like phase onto the magnetite surface for highest Co concentrations. These results might not only help predicting behavior and fate of Co in the environment, but also to optimize the synthesis procedures of Co-modified MNs using water as solvent for high technology applications