Molecular simulations with a resolved surface microstructure were performed for high-temperature dissociated oxygen reacting with a carbon-carbon composite material. The direct simulation Monte Carlo method was used to simulate the convection and diffusion of reactants toward the microstructure and the transport of surface reaction products away from the microstructure. Simulations were performed with and without gas-phase chemical reactions to determine the relative importance of gas-surface reactions compared to gas-phase reactions next to the material surface. The simulations incorporated reaction probabilities for individual gas-surface collisions based on new reactive scattering data obtained from molecular beam experiments. The experiments provide detailed dynamical data on the scattering of O, O2, CO, and CO2 from a representative material sample, made of vitreous carbon. These experiments indicate that a majority of surface reaction products were produced through thermal mechanisms. For a gas-surface temperature of 800 K, it was found that despite CO being the dominant surface reaction product, a gasphase reaction forms significant CO2 within the microstructure region. The amount of CO2 production within the microstructure region was shown to be dependent on the local Knudsen number, based on the exposed microstructure height. Finally, preliminary simulations were performed for a real carbon-carbon surface. The surface topography is obtained through X-ray microtomography of an ablated carbon-carbon sample, which is triangulated and used directly within a direct simulation Monte Carlo simulation of the gas-surface interaction.