Communication: Monolayer coatings on magnetic nanoparticles are of special interest because of their important technological applications in electronics, magnetic recording, magnetic fluids, etc. The surfactant coating acts as a stabilizer and is believed to prevent agglomeration. In this study the authors show that the self-assembly of the monolayer coating takes place on the surface of agglomerates, rather than on the surface of separate nanoparticles. This result is confirmed by elemental analysis and surface area and magnetic measurements of samples with different chain lengths of the surfactant molecule.