Thermal decomposition of the pesticide, chlorpyrifos (CPf) and its major degradation product, 3,5,6-trichloro-2-pyridinol (TCpyol), has been studied by quantum chemical calculation using density functional methods at the M06-2X/GTLarge//M06-2 X/6-31+G(d,p) level of theory. Chlopyrifos was found to undergo a series of unimolecular stepwise elimination reactions releasing two molecules of ethylene and finally HOPOS to form TCpyol. TCpyol underwent oxidative decomposition initiated by abstraction of its phenolic H atom by O2. Two phenoxy radicals so produced underwent combination leading to the formation of 2,3,7,8-tetrachloro-[1,4]dioxinodipyridine (TCDDpy). Via Smiles rearrangement both cis and trans TCDDpy are formed. Kinetic models have been constructed to model the decomposition of CPf into TCpyol and of the latter into cis and trans TCDDpy. Modeled results are compared with the experiments of Sakiyama et al. ( Organohalogen Compounds, 2012, 74, 1441-1444).