The regeneration mechanism of sulfur species formed on the Mg-Al-Ce-Fe mixed-spinel sulfur-transfer additives is clarified in this paper. Sulfate is the main sulfur species on the sulfur-transfer catalysts after the oxidative adsorption of SO2, and the S-O bond within sulfates shows lower stability in spinel phase than in the MgO phase. The reduction of these sulfate species leads to the emission of SO2 as well as H2S, and the formation of the two reductive products are both correlated with the change in Mg/Al ratio; however, SO2 is much more sensitive. When the regeneration was conducted at 550 degrees C, only H2S was observed as a reductive product; yet, at 700 degrees C, the small amount of H2S was preceded by a large amount of SO2. The mechanism for the sulfate reductive decomposition has been proposed that sulfite is the intermediate, which can be pyrolyzed into SO2 or undergo redox reaction to form H2S. The pyrolysis reaction is closely related to the relative magnitudes of the energy provided and required to break the S-O bond in sulfates, while the probability of the redox reaction is more likely to be dependent on the H-2 concentration. Sulfur-transfer additives with more surface-active sites should be developed to enhance the reductive ability, since more time is needed for the bulk-like sulfate to be reduced.