A short survey of some diffusion-controlled reactions has been given, such as termination reactions, formation of monomer from polymers at high temperatures and reaction of N0 2 gas with thin films stacked together. Further, poly-α-methylstyrene has been thermally degraded in absence of air in several solvents of different viscosities at 210°, 220° and 240°. The solvents were cyclohexanol, which has a high viscosity, cyclohexane and a 1:1 molar mixture of these two solvents. The degradation process consists of random initiation, depropagation and termination. In the very initial stages of the degradation, the termination consists of a reaction between polymer radicals, while somewhat later a termination reaction between a polymer radical and a monomer molecule is operative. At a certain critical chainlength, the kinetic chainlength becomes larger than the polymer chains and termination is eliminated. The experimental rate constants for chain scission and monomer formation have been found to be directly proportional to the reciprocal solvent viscosities. This indicates that the overall reaction is diffusion controlled. All experimental facts agree with the assumption that the rate determining step is the diffusion of polymer radicals out of cages, after a potential chain scission has taken place forming an encounter pair in a cage. This escape is quite a rare event. Diffusion constants, radii of gyration of the polymer radicals, rate constants for chain scission (escape from a cage), cage concentration and equilibrium values for equilibrium constants for cage formation from the polymer and its reverse reaction have been calculated. Arrhenius equations for the various experimental rate constants have been derived.