Monodisperse magnetic nanoparticles have generated huge interest in applied research within the last years. The control of their monodispersity and surface properties leads to a variety of nanotechnological applications. The use as non-volatile data storage media and sensor applications are in the focus of industry. Moreover, magnetic nanoparticles are close to be employed in tumor therapy, bio-labelling or contrast agents in magnetic imaging. To an increasing extend these are studied in fundamental research as well, since nanoparticles with diameters of a few nm bridge the gap between atomic and solid state physics. In particular, the intrinsic magnetic properties of fine particles such as the magnetocrystalline anisotropy, the saturation magnetization and the Curie temperature are affected by the reduction of their size. The subject of this thesis is the investigation of magnetic Co/CoO core-shell nanoparticles and metallic Co nanoparticles with diameters in the range of 9-14 nm. The nanoparticles have been prepared by means of organometallic synthesis, and they exhibit a high degree of monodispersity (σ < 10%).