Material properties and oxidation behavior of low-density felts used as substrates for conformal carbon/ phenolic ablators were compared with those of a rigid carbon fiber preform used to manufacture heritage lightweight ablators. Synchrotron X-ray micro-tomography measurements were performed to character- ize the materials’ microstructure at the scale of the fibers. Using the tomography voxels as computational grids, tortuosity in the continuum regime, and room temperature conductivity were computed. Micro- scale simulations of the oxidation of carbon fibers were carried out using a random walk model for oxy- gen diffusion and a sticking probability law to model surface reactions. The study shows that, due to a higher porosity and lower connectivity, the felt materials have lower thermal conductivity but a faster recession rate than that of the rigid preform. Challenges associated with computations based on micro-tomography are also discussed.