This study investigates the soot oxidation activity and mechanism of an Al2O3-supported Ag2SO4 catalyst with novel active species. The Ag2SO4/Al2O3 catalyst exhibits higher soot oxidation activity under loose contact conditions compared to Ag/Al2O3 and Ag/CeO2 catalysts. Moreover, it is hardly affected by sulfur poisoning and maintains its high activity after sulfur poisoning treatment. High-temperature X-ray diffraction measurements suggest that a part of Ag2SO4 on the Al2O3 support melts and creates contact between the Ag2SO4 and soot at temperatures over 400 degrees C, resulting in higher soot oxidation activity even under loose contact conditions. The soot oxidation mechanism over the Ag2SO4/Al2O3 catalyst is probed by oxygen isotope exchange analysis and operando X-ray absorption fine structure measurements, which reveal that the oxygen chemically bound in Ag2SO4 is utilized for soot oxidation. In particular, the above oxidation features a reversible redox cycle comprising partial reduction of Ag2SO4 to metallic Ag that is further re-oxidized to Ag2SO4 by the gas-phase oxygen.