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The influence exerted by the initial thickness on the thermal degradation of PMMA has been studied experimentally. The mass loss rate and the temperature profile in the solid have been measured continuously for .6, 1.0, 1.5, 3.0 and 5.0 cm thick specimens. All the experiments have been performed under nitrogen. The evolution of the thermal degradation has been interpreted in term of the two variables R=mâ€(Q-Q1) mass loss rate per unit heat flux absorbed by the material and qâ€=f^t(Qâ€-Qâ€1) amount of heat accumulated0 into the solid. Two conclusions have been drawn from the experimental results: -there exists a critical thickness below which the maximum mass loss rate depends on the initial thickness of the specimen. -since this variation results from a change in the heat loss from the backside, it does not affect the curve R=f (qâ€) except for very thin materials.