Heating hydrous manganese (II) hydroxide gel at 85• C for 12 hours produces Mn 3 O 4 nanoparticles. They were characterized by X-ray powder diffraction (XRD) and infrared spectroscopy (FTIR). The particle size estimated from the SEM and X -ray peak broadening is approximately 32 nm, showing them to be nanocrystalline. EPR measurements confirm a typical Mn 2+ signal with a highly resolved hyperfine structure.