The use of a rule-based automated kinetic model builder Genesys is illustrated for the thermal decomposition of diethyl sulfide and ethyl methyl sulfide. Rule-based automatic kinetic model generation builds-upon the users’ expert knowledge to define constraints per reaction family to limit the model size and exclude species and reactions that are considered irrelevant. In the case of alkyl sulfide pyrolysis intermolecular hydrogen abstractions, intermolecular additions/β-scissions and intermolecular homolytic substitutions are used to iteratively expand the model not considering species with more than 5 heavy atoms. Furthermore, the formation of biradical species and cyclic structures was avoided. Rate coefficients of elementary reactions and thermochemical properties of molecules were estimated through group additive methods, with parameters solely derived from high level ab initio calculations. 39 reactions were added to the model after the automatic generation out of which 37 got rate constants assigned from ab initio calculations. The generated model, consisting of 444 reactions between 28 molecules and 38 radical species, was validated using experimental data for the thermal decomposition of diethyl sulfide and ethyl methyl sulfide and showed that measured and predicted species mole fractions are in good agreement. However, there is a clear need for more accurate and more detailed experimental data for the pyrolysis of sulfur containing compounds.