Three samples of passivated ultrafine iron carbide particles, synthesized by laser-induced pyrolysis of gaseous precursors, were examined by X-ray diffraction at temperatures as high as 600 degrees C under hydrogen and helium atmosphere to study the effect of gaseous treatments on structural and phase transformations. The approximate 6-7-nm-diameter particles began to sinter at temperatures between 100-300 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, structure, and gaseous treatments. This topotaxy leads to the selective formation of gamma-Fe2O3 from specific ultrafine particle carbides on exposure to helium at high temperature and to the reduction of carbides to alpha-Fe on exposure to hydrogen at high temperature. The characteristic reduction of carbides to alpha-Fe on exposure to hydrogen at high temperature. The characteristic reaction paths are attributed to the nature of synthesis and the subsequent passivation of the iron carbide powders.