The synthesis of iron oxide particles was carried out by the hydrolysis of iron tri-n-butoxide in an octanol/acetonitrile solution. The morphology of the particles formed was found to depend on composition of the mixture of solvents used and also on the method of its addition, i.e. either instantaneous or continuous. The data of apparent reaction rate revealed that the selection of reaction conditions controlling the reaction rate was the most important factor for achieving formation of monodispersed particles. These particles were subsequently used for preparation of magnetic particles by de-oxidation process.With respect to the relationships between the reduction condition and magnetic characteristics of the particles formed in this process, it was found that the particles synthesized showed a saturation magnetization magnetic susceptibility greater than that of commercial magnetite when they were reduced at a temperature higher than 573 K under a hydrogen atmosphere. The particles obtained after reduction treatment at 523 K for 3 h were identified as gamma-Fe2O3 and those at temperature higher than 623 K for 3 h as alpha-Fe types, respectively, by using powder Xray diffraction technique.A suspension obtained using the monodisperse magnetic particles in silicone oil was prepared and its rheological characteristic was determined using a parallel-plate rheometer and was found to have an apparent magnetorheological (MR) effect. It is believed that the monodisperse magnetic particles obtained in this work can be used as a component of a model MR fluid to elucidate the general mechanism of cluster formation of particles in such fluids. (C) 2003 Elsevier Science B.V. All rights reserved.