We have investigated the continuous hydrothermal. synthesis and crystallization of spinel CoFe2O4 via the reaction of ferric nitrate and cobalt nitrate with sodium hydroxide. The reaction was carried out in water at temperatures ranging from 475 to 675 K and pressures of 25 MPa. The relative solubility of the precipitating cations was found to play a critical role in attaining the correct product. It was found necessary to control pH and temperature in order to prevent premature precipitation of iron in the reactor. Two variations of the continuous hydrothermal technique were examined-cold mixing and hot mixing. The cold mixing experiments produced a product with less impurity than the hot mixing experiments. Furthermore, the cold mixing configuration was successful in producing uniform nanoparticles of CoFe2O4. A mechanism of particle formation was postulated involving the precipitation of metal hydroxides at ambient conditions, dissolution of the hydroxides as temperature is increased followed by rapid precipitation of metal oxides at elevated temperatures. The hot mixing experiments, on the other hand, simply involve the precipitation of metal oxides due to the addition of the hot hydroxide solution. In both cases, very fine particles of CoFe2O4 are produced in the range of the processing conditions investigated. (C) 2003 Elsevier Science B.V. All rights reserved.