The destruction of stockpiles or unexploded ammunitions of bis(2-chloroethyl) sulfide, also called mustard gas or yperite, by thermal treatments requires the development of highly safe processes. The high-level of toxicity of this compound induces a high level of complexity for any experiments. Consequently, there is a considerable lack of knowledge on the behavior of this chemical under high-temperature conditions (with or without oxygen). In this work a detailed chemical kinetic model for the combustion and pyrolysis of mustard gas is proposed for the first time. A large number of thermo-kinetic parameters were calculated using quantum chemistry and reaction rate theory. The model was validated against experimental pyrolysis data of the literature. It was shown that the degradation of mustard gas is ruled by a chain reaction mechanism where the chlorine atom is the principal chain carrier. HS radical, formed in the primary mechanism by an original pathway found using quantum calculations, was also proved to be an important chain carrier. Comparison with the kinetics of the usual simulant of mustard gas, diethyl sulfide, showed that the lack of chlorine atom in the former chemical leads to an inappropriate simulation of the mustard gas behavior. Combustion and pyrolysis simulations were also compared and surprisingly demonstrated that chlorine atoms remain the main chain carrier in combustion.