Nanometer-sized iron oxide powder was prepared by reaction of iron pentacarbonyl (Fe(CO)(5)) and oxygen in a gas phase reactor. X-ray diffraction and differential thermal analysis revealed that depending on the reaction temperature of alpha-Fe2O3 (temperature > 600 degreesC) or the thermodynamically less stable gamma-Fe2O3 (temperature < 600 degreesC) was formed. The surface area of the gamma-Fe2O3 was in the range 100-240 m(2) g(-1). By reducing the reaction pressure, the surface area of the powders produced was increased. High precursor concentrations also favored the formation of powders with higher surface areas. While the pressure effect can be explained by the growth time of the particles, the slower particle growth at higher initial Fe(CO)(5) concentration is unexpected and cannot be explained by diffusion-controlled particle coagulation. Transmission electron microscopy of gamma-Fe2O3 revealed in accordance with area measurements primary particle diameters below 10 nm. (C) 2002 Elsevier Science B.V. All rights reserved.