Fe3O4 particles prepared by oxidation of Fe(OH)(2) precipitated from FeCl2 solutions containing different amounts of Ni2+ from 0 to 30 mol% were examined by various means. The thermogravimetry (TG) results show that the addition of Ni2+ raised the air-oxidation temperature of Fe3O4 particles from ca. 200 to ca. 400 degrees C, which means that the Fe3O4 particles were stabilized to air-oxidation by the doped Ni2+. Fe2+ content of the materials obtained at Ni/(Ni + Fe) greater than or equal to 0.1 mol% was significantly greater than the theoretical Fe2+ content of Fe3O4. Fe-57 Mossbauer spectra indicated that Fe3+ ions in B sites of the Ni-doped Fe3O4 are exchanged in part by Fe2+. The de electrical conductivity was increased from 10(-4) to 10(-2) S cm(-1) by Ni2+ loading.