Nanoparticles of Fe3O4 were synthesized by hydrolysis in an aqueous solution containing ferrous and ferric salts at various ratios with 1,6-hexanediamine as a base. It was found that the ferrous to ferric ratio influences the reaction mechanism for the formation of Fe3O4. When the ratio of ferrous to ferric ions was increased, the formation of large hydroxide particles as a precursor of Fe3O4 was promoted, which resulted in an increase in the size of Fe3O4 nanoparticles. As a result, the mean diameter of Fe3O4 nanoparticles increased from similar to 9 to similar to 37 nm as the molar percentage of ferrous ions with respect to the total iron ions was increased from 33 to 100%. Furthermore, it was demonstrated that magnetic properties of Fe3O4 nanoparticles can be controlled by adjusting the molar ratio of ferrous to ferric ions as well as the particle diameter. (C) 2007 Elsevier Inc. All rights reserved.