Detailed numerical modeling of the surface oxidation of polypropylene ( PP) films was performed to define the reactions contributing to the oxidation of the film. The surface reaction mechanism developed by Dorai and Kushner provided the basic framework for the chemistry used in the model. The calculated results correlate well with available experimental data on PP surface atomic oxygen to carbon ratios and provide insight into the pathways of polymer surface oxidation. The results show that O-2 and OH play major roles in the modification of the surface and that the rate limiting step is the abstraction of hydrogen by the hydroxyl radical. As the flame equivalence ratio changes, the dominant surface functional groups change from the peroxy group under lean conditions, to a combined contribution of the peroxy, alkoxy, and hydroxyl groups at stoichiometric, to the hydroxyl surface species alone under fuel- rich conditions.