Iron oxide films have been prepared by thermal decomposition of iron pentacarbonyl vapor in an oxidizing atmosphere. Optical spectra show these to be opaque in the ultraviolet but partially transparent to visible light. The nature of these deposits was studied by electron and x‐ray diffraction, electron microscopy, and the Mössbauer effect. It was found that deposition at low temperatures resulted in a partially crystalline film, while films formed at higher temperatures were composed of crystallites. The chemistry of the deposition process was considered in light of mass spectrometer analyses of species present during deposition.