The relative dissolution rate is important because it determines the mechanism of the particle formation. It is determined by the solubility and particle size of the reactants. In the preparation of LiFe5O8 by the reaction between Li2CO3 and Fe2O3 in Li2SO4-Na2SO4 salt, Li2CO3 dissolves completely in the molten salt and LiFe5O8 particles form by the solution- diffusion process (mechanism 2) (Wickham, 1971). Acicular NiFe2O4 and ZnFe2O4 particles are prepared by the reaction between acicular Fe2O3 and equiaxed NiO and ZnO using NaCl-KCl and Li2SO4-Na2SO4 (Hayashi et al., 1986a). The particles obtained in the reaction stage (700ºC for 1 h) are divided into two groups; one has the acicular shape and the other has a deformed shape with equiaxed grains of about 0.1 μm and rounded acicular particles. ZnFe2O4 obtained in NaCl-KCl and Li2SO4-Na2SO4 and NiFe2O4 obtained in NaCl-KCl have the acicular shape, whereas NiFe2O4 obtained in Li2SO4-Na2SO4 has the deformed one. The effect of the chemical species on the particle shape is explained by the solubility of ferrites in molten salt (Table 1). NiFe2O4 has the highest solubility in Li2SO4-Na2SO4 (5.1×10−7 mol/g salt) compared to NiFe2O4 in NaCl-KCl (0.98×10−7 mol/g salt) and ZnFe2O4 in Li2SO4-Na2SO4 and NaCl-KCl (1.6×10−7 and 1.8×10−7 mol/g salt, respectively). The possible explanation is that the high solubility of NiFe2O4 in Li2SO4-Na2SO4 requires an extensive time for saturation with NiFe2O4. This gives a greater opportunity for Fe2O3 to dissolve, and NiFe2O4 particles are formed by the solution-precipitation process (mechanism 1).