Applications of nanoparticles in biology require that the nanoparticles remain stable in solutions containing high concentrations of proteins and salts, as well as in cell culture media. In this work, we developed simple protocols for the coating of sub-10 nm nanoparticles and evaluated the colloidal stability of dispersions in various environments. Ligands (citric acid), oligomers [phosphonate-terminated poly(ethylene oxide)], and polymers [poly(acrylic acid)] were used as nanometer-thick adlayers for cerium (CeO2) and iron (gamma-Fe2O3) oxide nanoparticles. The organic functionalities were adsorbed on the particle surfaces via physical (electrostatic) forces. Stability assays at high ionic strengths and in cell culture media were performed by static and dynamic light scattering. Of the three coatings examined, we found that only poly(acrylic acid) fully preserved the dispersion stability over the long term (longer than weeks). The improved stability was explained by the multipoint attachments of the chains onto the particle surface and by the adlayer-mediated electrosteric interactions. These results suggest that anionically charged polymers represent an effective alternative to conventional coating agents.