Nanosize CoFe2O4 particles prepared by a microemulsion method have been investigated by X-ray diffractometry, transmission electron microscopy. SQUID magnetometry and Mossbauer spectroscopy. All peaks of X-ray diffraction patterns can be attributed to a cubic spinet structure with the lattice constant a(0) = 8.397 Angstrom. The average size of the particles, determined by transmission electron microscopy, is 49 Angstrom. The coercivity and the maximal magnetization measured at 5 K in an applied magnetic field of 50 kOe are 15.1 kOe and 15.2 emu/g, respectively. In the same field of 50 kOe, the measured blocking temperature is about 35 K. Superparamagnetic behavior of these particles around room temperature is confirmed by the coincidence of the magnetization (M) vs. an applied magnetic field (H/T) plots for different temperatures and the Mossbauer spectra. (C) 2001 Elsevier Science B.V. All rights reserved.