Three samples of passivated ultrafine iron carbide particles, synthesized by laser induced pyrolysis of gaseous precursors, were examined by x-ray diffraction, x-ray photoelectron spectroscopy, and thermogravimetry/mass spectroscopy at temperatures as high as 600-degrees-C under a He atmosphere. The approximate 6-7 nm diam particles began to sinter at temperatures between 300 and 400-degrees-C and formed a complex mixture of carbide, oxide and metallic phases. Preservation of the carbide structure on heating was dependent on the purity of the carbide, and the concentration of oxygen and carbon in the particles. A topotactic transformation from Fe7C3 to Fe0.98O was observed and is discussed relative to the as-synthesized/passivated particles composition and structure. This topotaxy leads to the selective formation of gamma-Fe2O3 from specific ultrafine particle carbides.