Redox synthesis of magnetite powder from metallic iron and FeO(OH) under mixing-grinding with the assistance of H2O2 as an oxidizing reagent has been proposed. Deformability of the metal-oxide system induces the kneading of am-FeO(OH) into metallic iron to activate, while the iron is oxidized to Fe(OH)(2) and/or FeO by H2O2 Contained in the grinding liquid (acetone). Only the reaction product, gamma-Fe3O4, is obtained. The adhesivity of the powder system due to iron reduces with the progress in synthesis reaction forming non-deformable magnetite powder to result in its dispersion into acetone. The primary particle size of the gamma-Fe3O4 powders formed is less than 100 nm. The saturation magnetization of the synthesized gamma-Fe3O4 is 9.24 x 10(-5) Wb m g(-1), which is about 90% of the commercially available fine gamma-Fe3O4 powders with needle-like shapes. (C) 2004 Elsevier B.V. All rights reserved.