We are reporting on the synthesis of Mn3O4 nanoparticles and CTAB-Mn3O4 nanocomposites via a sonochemical route using MnCl2, ethanol, NaOH and CTAB. The crystalline phase was identified as Mn3O4. The crystallite size of the CTAB-Mn3O4 nanocomposite was identified as 13 +/- 5 nm from X-ray line profile fitting and the particle size from TEM was 107.5 +/- 1.4 nm. The interaction between CEAB and the Mn3O4 nanoparticles was investigated by FTIR and H-1 NMR spectroscopies. Two different magnetic phase transitions were observed for both samples below the Curie temperature (43 degrees C) by using a low temperature Electron Paramagnetic Resonance (EPR) technique. Also we determined the effect of the capping with CTAB on the reduction in absorbed power. (C) 2010 Elsevier Ltd. All rights reserved.