Understanding the adsorption of amino acids (AAs) onto magnetic iron-oxide nanoparticles (SPIONs) is important not only for the preparation of the aqueous suspensions, but also for understanding the interactions at the bio-nano interface. In this investigation the adsorption of aspartic acid (Asp) and lysine (Lys) onto SPIONs was studied, based on a characterization of the suspension properties, i.e., measurements of the ξ-potential, the hydrodynamic size and the osmolality, and by direct HPLC analysis of the AA in the supernatants and at the nanoparticles of the ultra-centrifuged suspensions. The results show that the AAs adsorb onto the SPIONs in the form of large molecular associates, which decisively influence the nanoparticles' surface properties. A measurement of the freezing-point depression using a Knauer osmometer proved that the molecular associates are already formed in the AA aqueous solutions.