Iron-oxide (Fe2O3) nano-particles in the size range 4 to 20 nm have been synthesized via a gas-phase route utilizing a H-2/O-2/Ar low-pressure flame. The burner stabilized flame was doped with different amounts of iron-pentacarbonyl (Fe(CO)(5)) with concentrations in the range 262 to 2096 ppm. The particles were analyzed in situ according to their mass and charge by means of a particle mass spectrometer (PMS). The specific surface area, size. and structure of collected particles were determined ex situ by BET-adsorption and transmission electron microscopy (TEM). The particles are single crystals of gamma -Fe2O3 as identified by X-ray powder diffraction and Mossbauer spectroscopy. The experimental results were verified by a theoretical model including the homogeneous gas-phase reactions of H-2/O-2/Ar as well as transport properties of the burner stabilized flame and the particle dynamics. Good agreement of measured and calculated particle sizes was obtained.