Ultrafine, magnetic nickel ferrite (NiFe2O4) particles of high crystallinity were prepared by hydrolysis of organic iron(III)-nickel carboxylate solutions with water at 200-240 degrees C and 0.75-1.3 MPa at reaction times between 10 and 120 min. The resulting nickel ferrite was free from contamination by the organic material, and the particle size drastically decreased from 0.5 to 0.05 mu m as the stirring intensity of the starting solution increased. The rate of precipitation of the nickel ferrite was markedly dependent on process conditions such as temperature, pressure, stirring intensity, and organic-phase composition.