A quant. kinetic model is described, which provides insights of the inner mechanisms that play a key role in the natural oxidative degrdn. (photoinduced and thermal) of a model polymer, polyethylene oxide (PEO). A set of key reactions is selected from ab initio data, theor. rate const. calcns., and exptl. rate consts. Exptl. 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 degrdn. 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., intermol. and γ-intramol. H-abstraction reactions, and the termination reaction. The competition between these processes is the driving force of the degrdn. mechanism. In thermo-oxidative conditions, the relative quantities of esters and formates depend strongly on four competitive hydroperoxide decompn. channels, i.e., the unimol., bimol., induced decompn. by hydroxyl radicals, and the hydroperoxide H-abstraction reaction. Many of the reactions studied participate also in polymer synthesis, pyrolysis or other degrdn. processes. The simple structure of the model polymer is representative of a wide range of polymer systems.