This study investigates the thermochemical data and rate parameters for facile routes involved in combustion of fossil fuels, particularly in case of coal fires, using the density functional theory (DFT). We have studied mechanisms for the decomposition and oxidation reactions of cyclic organosulfur compounds including tetrahydrothiophene, 2-methyltetrahydrothiophene, 2,5-dimethyltetrahydrothiophene and 1,4-thioxane. The results show a pleasing consistency and considerably expand the available data for these important compounds. CBS-QB3 based calculations predict that thiophene family decomposes via two types of bond breakages, namely, C-S and C-C β bond-scission with C-S bond cleavage to be the preferred pathway on account of lower barrier heights. However, reaction of a model compound from thiophene group with O 2 offers significantly large barriers (i.e., > 20 kcal/mol) indicating that direct oxidation of thiophenes by ground state oxygen might be important only in case of high temperature combustion processes. Decomposition of 1,4-thioxane (TO) is observed to be favoured by ring opening processes with β-CH 2 -O scission, based on the DFT calculations. Further decomposition of thioxane radicals leads to two heavy atom species and generation of doubly unsaturated four heavy atom segments with reasonable endothermicity. Although TO radicals react readily with oxygen, the resulting intermediate formed requires considerable activation barriers to be overcome for ring opening reactions. The implication of current theoretical results sheds new light on further pyrolytic and oxidative mechanisms for these particular compounds.