MnZn ferrite (Mn0.5Zn0.5Fe2O4) nanoparticles in amorphous SiO2 were prepared using the chemical wet method. On the basis of the X-ray diffraction peaks, the particle sizes were estimated to be between 17 and 41 nm, depending on the flow rate of Ar gas. The particle size increased with the flow rate of Ar gas. From magnetization measurements, the magnetization values varied in accordance with the conditions of Ar gas. To clarify the electronic state of metal ions of nanoparticles, X-ray absorption fine structure (XAFS) measurements were performed, and the results were compared with X-ray diffraction patterns. Copyright © 2010 John Wiley & Sons, Ltd.