Nanocrystalline Mn3O4 has been synthesized by ultrasonic irradiation of Mn acetate solution in water. Analysis of its X-ray diffraction data shows formation of a phase-pure compound with an average particle size of about 15 nm. DC magnetization measurements as a function of temperature and field show a reduced ferrimagnetic transition temperature (T-c approximate to 39 K) as compared to those reported for the bulk (T-c approximate to 43 K), and a subsequent observation of superparamagnetic behavior at 40 K. The observed magnetic properties are suggestive of formation of a single domain magnetically ordered Mn3O4 nanoparticles below their ferrimagnetic transition temperature. (c) 2005 Elsevier B.V. All rights reserved.