The quantitative kinetic model presented in this work provides insights of the inner mechanisms that play a key role in the natural oxidative degradation (photoinduced and thermal) of a model polymer, polyethylene oxide (PEO). A set of key reactions is selected from ab initio data, theoretical rate constant calculations, and experimental rate constants. Experimental results, i.e., induction times and quantities of carbonyl end-products, are accurately reproduced by this model. While this study grasps the complexity of the degradation mechanism, results depend mainly on the reactivity of peroxy radicals and that of hydroperoxide groups. Three pathways are available to the peroxy species, i.e., intermolecular and γ-intramolecular H-abstraction reactions, and the termination reaction. The competition between these processes is the driving force of the degradation mechanism. Moreover, in thermo-oxidative conditions, the relative quantities of esters and formates depend strongly on four competitive hydroperoxide decomposition channels, i.e., the unimolecular, bimolecular, induced decomposition by hydroxyl radicals and the hydroperoxide H-abstraction reaction. Many of the reactions investigated here participate also in polymer synthesis, pyrolysis or other degradation processes. Moreover, the simple structure of the model polymer in this work is representative of a wide range of polymer systems.