Dimethyl sulfoxide (DMSO) is widely used due to its excellent solvent properties; however, several serious incidents have been reported related to the use of DMSO. DMSO can decompose autocatalytically, but we have no consensus with respect to the chemical structure and function of the autocatalyst. Thermal decomposition of DMSO was carried out in an inert atmosphere, and analysis of the nonvolatile fraction indicated the presence of several organic and inorganic acids. DMSO was believed to act as an oxidizer as well as a reactant in the formation of these acids. The same acids were found in an isothermal heating test, and their concentrations increased over heating time. Addition of acids to DMSO before starting isothermal heating significantly shortened the induction period indicating that the acids generated in situ served as autocatalysts for the thermal decomposition of DMSO.