Nanosized amorphous NiFe2O4 powder was prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO)(5) and Ni(CO)(4), in decalin at 273 K, under an oxygen pressure of 100-150 kPa. The amorphous nature of these particles was confirmed by various techniques, such as SEM, TEM, electron microdiffraction, and X-ray diffractograms. Magnetic measurements, Mossbauer, and EPR spectral studies indicated that the as-prepared NiFe2O4 ferrite particles were superparamagnetic. The Mossbauer spectrum of the crystallized sample showed a clear sextet pattern, with hyperfine field values of 500 and 508 kOe for A (tetrahedral) and B (octahedral) sublattices, respectively, of the inverse spinel NiFe2O4. Saturation magnetization of the annealed sample (25 emu/g) was significantly lower than that for the reported multidomain bulk prticles (55 emu/g), reflecting the ultrafine nature of the sample. Thermogravimetric measurements with a permanent magnet gave Curie temperatures of 440 degrees C for amorphous and 560 degrees C for the crystallized forms.