Monodisperse Co, Fe, and FeCo nanoparticles are prepared via thermal decomposition ofmetal carbonyls in the presence of aluminium alkyls, yielding air-stable magnetic metalnanoparticles after surface passivation. The particles are characterized by electronmicroscopy (SEM, TEM, ESI), electron spectroscopy (MIES, UPS, and XPS) and x-rayabsorption spectroscopy (EXAFS). The particles are peptized by surfactants to form stablemagnetic fluids in various organic media and water, exhibiting a high volume concentrationand a high saturation magnetization. In view of potential biomedical applications of theparticles, several procedures for surface modification are presented, includingpeptization by functional organic molecules, silanization, and in situ polymerization.