Nanosized Mn3O4 single crystals are prepared using the polyol method and characterized with powder X-ray diffraction, transmission electron microscopy and physisorption experiments. Depending on the reaction time, either oval-like nanocrystals with a mean TEM size of 7.9 nm or rhomboedron-like particles with a mean TEM size of 12.6 nm are isolated. These particles are active towards the decomposition of aqueous hydrogen peroxide at room temperature and the data show that the reaction kinetics depend on the particle size and shape. The degradation of a model substrate, methylene blue is also investigated. (C) 2010 Elsevier B.V. All rights reserved.