Pyrolysis-gas chromatography has been used to locate the degradation front in pieces of typical phenolic ablative materials. This technique gives quantitative results for per cent phenolic resin versus distance normal to the surface. The charred material is carefully machined so as to remove successive 0.020-in. cuts starting with the charred face of the material. Each cut is then analyzed by the combined techniques of pyrolysis and gas chromatography. A plot of per cent phenolic resin versus distance from the undecomposed back face gives a profile of the degree of degradation of the ablative plastic. Samples are weighed on the pyrolysis ribbon before and after pyrolysis giving total per cent volatiles. Divergence of the per cent volatiles and per cent phenolic resin curves near the charred surface of the material is due to the presence of the pyrolysis products near the surface.