Chemical industry produces and uses high sulfuric acid volumes, and many processes generate high spent acid (ASS)volumes. The use of ASS as starting materials contributes to the protection of the environment. Should; this alternative not be implemented, ASS volumes could represent high amounts of wastes. The thermal oxidative cracking process starts with ASS to produce commercial grade sulfuric acids. The presence of organic and mineral pollutants in;spent acids can greatly influence the conversion yield, and this study addresses this point. The thermal decomposition of sulfuric acid is studied in a high-temperature plug flow reactor; the influence of temperature, oxygen concentration, and sample concentration on SO2 formation has been quantified. It is shown that the two last parameters have only a small influence on sulfur dioxide yield. Temperature is the main factor because of its influence on the kinetics of the conversion of SO3 to SO2. The second part of the study is related to the influence of the presence of contaminants: nitric acid and butanol. It is shown that the decomposition of HNO3 enhances SO3 conversion to SO2, the decomposition of the hydrocarbon also increases sulfur dioxide yield.