In the general theory of chain depolymerization reactions two parameters are important: the kinetic chain length 1/ε and the transfer constant σ. In terms of these the molecular weight distribution, average molecular weights and rates of decomposition can be discussed. These rates decrease monotonously with conversion when the kinetic chain length is large in comparison with the D.P., regardless of the initiation mechanism. When it is small, there is a maximum rate, if the initiation is random or transfer is pronounced. If initiation is occurring only at the chain ends, the reaction becomes of zero order. Although the general theory for any value of the kinetic chain length has been developed, numerical solutions are so far available only for extreme cases. However, the initial rate has been given for all conditions. The theory is compared with experimental results on molecular weight change and rate of degradation for polystyrene and polymethylmethacrylate, respectively. In the former instance, an estimate of the kinetic chain length is made from the observed monomer yields. In the latter, ε and the rate constant for initiation are obtained from molecular weight and rate curves. The observed molecular weight decrease in methyl methacrylate is more pronounced for the larger molecular weights and higher degrees of conversion, than the theory neglecting chain transfer would predict. This has been tentatively ascribed to chain transfer becoming more important at later stages of the reaction. Thus, it would appear that the general state of affairs in thermal depolymerization of addition polymers is accounted for.2 However, discrepancies remain which can at least qualitatively be explained in the framework of the general theory. Their quantitative interpretation requires a refinement of the reaction mechanism considered, in particular the initiation step, and further numerical evaluation of the rate equations. The theory was developed in order to interpret the thermal and photodecomposition of chain polymers. There are degradative processes which exhibit a striking similarity to the phenomena discussed here, although the detailed chemistry is, of course, entirely different. In the deterioration of textiles, some agents attack the fabric in such a way, as to leave the tensile strength of the residue unimpaired, while others produce a gradual decay in the tensile properties. Also, the breakdown of proteins by proloolytic enzymes seems to occur principally in two ways, one leaving intact material and small species, the other producing molecules of intermediate sizes.3 A kinetic description of these reactions could be similar to ours, which involves, in one extreme a slow process, followed by more rapid ones, and in the other a single, moderately rapid step.