Soot formation over zinc ferrite high-temperature desulfurization sorbents was examined. When active zinc ferrites were used in simulated coal-derived gases for fuel cells, a large amount of soot was formed over the ferrites. This soot formation decreased with the increase of reaction temperature. This soot formation was inhibited by H2O and CO2 and accelerated by H-2 and CO. These dependencies of the rate of soot formation on the concentration of H2O, CO2, H-2, and CO can be expressed by six experimental equations. Carbides (Fe3C- and Fe3C-like compound, FexC) were found in the sample over which soot was formed. Fibrous ZnO was formed over the surface of the sample when the sample ZnFe2O4 was converted to carbides (Fe3C and FexC). It is thought that the carbides and the soot were produced via a surface active carbon.