The intent of this work is to study the performance of a catalyst for soot combustion under reducing and oxidizing environments, to draw indications on the catalytic mechanism and on the implications of the sulfation process. Thermogravimetric (TG) analyses of the sulfated catalyst in reducing and oxidizing environments and on-line mass spectrometry (MS) analyses of the gaseous products of the TG analyses have been performed. The catalyst is able to reduce and oxidize, when treated in suitable environments. Both H-2 and carbon reduce the catalyst within comparable ranges of temperature, although the main products of the reductions are different. In addition, the temperature ranges within which the catalysts reoxidize after the reductions are practically coincident with the catalyst threshold temperature. The aforementioned findings have led us to address the catalyst reduction as the rate-limiting step of the soot oxidation process. Sulfation decreases the catalyst activity through two different mechanisms that involve different catalytic components. One, which is irreversible, is due to the chlorine substitution with sulfur, and the other, which is reversible, is likely related to the physical or chemical covering of some other catalytic compounds that do not contain chlorine initially.