Nanosized manganese oxide particles were prepd. by the so-called polyol process. The av. diam. of the particles was controlled by the growth time. XRD, TEM and x-ray photon spectroscopy (XPS) show that the particles are well crystd., pure, stoichiometric Mn3O4 single crystals of uniform size ranging from ~5 to 12 nm. The variation of their dc-magnetization, M, as a function of the magnetic field, H, and temp., T, clearly corresponds to ferromagnetic ordering at low temp., with a Curie temp. slightly >40 K The evidence for superparamagnetism in these particles, due to their very small size, is discussed in the light of their M(H) and M(T) for zero-field-cooled (ZFC) and field-cooled (FC) plots. [on SciFinder(R)]