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CoFe2O4 nanoparticles with diameter of about 5 nm were prepared at 390 degrees C by batch supercritical hydrothermal synthesis method. It was found that the characteristic of products depended on pH, temperature, mole ratio (r) of Co2+ to Fe3+ and coexisting cations in reactant solutions. The reaction time and the coexisting cations (Na+ and K+) had little effect on the size and morphology of CoFe2O4 particles. The products were a mixture of CoFe2O4 and Fe2O3 at r < 0.5 and Co1+xFe2-xO4 (0 <= X <= 1) at r >= 0.5. Pure CoFe2O4 nanoparticles with uniform size were synthesized at 390 degrees C, pH 12 and r = 0.5. The maximum coercivity and saturation magnetization of CoFe2O4 prepared by the present method were 340.6 Oe and 68.9 emu/g, respectively. The mechanism for batch supercritical hydrothermal synthesis of CoFe2O4 nanoparticles is believed to be a homogeneous phase reaction of Co(NO3)(2)center dot 6H(2)O and Fe(NO3)(3)center dot 9H(2)O aqueous solution. (c) 2007 Elsevier B.V. All rights reserved.