Magnetic fluids formed from uncoated and silica-coated cobalt-ferrite nanoparticles dispersed in water have been synthesized and characterized by means of transmission electron microscopy (TEM) and small angle neutron scattering of polarized neutrons (SANSPOL). Particle shape, size, size distribution of the cores, and shell thickness, as obtained from the above-mentioned techniques, are reported for both uncoated and silica-coated cobalt-ferrite nanoparticles. SANSPOL takes advantage of the modification of contrast for the two magnetic polarization states, making possible the separation from the scattering of the magnetic and nonmagnetic contributions of the nanoparticles, and allowing a complete characterization of magnetic and diamagnetic domains of nanoparticles. The superiority of the SANSPOL technique over TEM is highlighted.