In the present work, numerical computations of the flow and thermal fields have been carried out for a transient, two-dimensional model of thermal and oxidative degradation of polymethylmethacrylate (PMMA) subjected to a monochromatic, radiant heat flux. An external thermal radiation source is used to simulate the primary mode of energy transfer in a room fire. An incompressible SIMPLE code is used with a staggered grid arrangement. The equations for the fluid and solid (fuel) phases are solved simultaneously using a segregated technique. At the outlet of the computational domain, a convective boundary condition is compared with a traditional Neumann condition. The convective boundary condition is shown to be more effective in reducing the CPU time. A study in the effects of spatial resolution and different time steps are provided. A theory is developed to account for thermal and oxidative degradation. The theory is based on differences in polymer degradation behavior in inert and non-inert environments. A number of quantities such as surface temperature and mass flux of PMMA are calculated by an external source of 40 kW/m2. The predictions of the model are in a good agreement with the experimental results. It is found that an increase in gas-phase oxygen concentration obviously decreases the surface temperature and increases the gasification rate of PMMA.