The transient, one-dimensional, thermally thick, noncharring solid material version of a numerical thermal degradation model is used to analyze the thermal degradation process of poly(methyl methacrylate) when subjected to a radiation source from a graphite plate. A theory is developed to account for oxygen-sensitive thermal degradation, which is based on differences in polymer degradation behavior in inert and non-inert environments. The model includes condensed phase heat transfer, in-depth thermal and oxidative decomposition, advective mass transfer, and in-depth absorption of radiation. It is found that an increase in gas-phase oxygen concentration decreases the surface temperature and increases the gasification rate substantially. The predictions yield physically realistic results when compared with published experimental data for an external radiation source with flux of 17 kW·m m 2 .