A study is made of the thermal response of charring ablators to given environmental conditions. The heat protective materials considered form gases and a porous char as they undergo thermal degradation in depth. The analysis considers the transient conditions and the effect of porosity pressure on the gas pressure build-up within the porous char. The effects of porosity and permeability are included. Results of the temperature, material density, and gas pressure distribution through the heat protective layer are presented for a typical charring material. Two cases are considered: in the first case, the material undergoes one degradation reaction; in the second case, the material under goes two successive reactions. The inclusion of char removal can also be accomplished by this study.