Abstract A study was made of the kinetics of oxygen absorption and isothermal weight change as a function of layer thickness for molten polyethylene at 157° under oxygen at 650 torr pressure. The effect of the rate of transport processes on the rate of thermal oxidation was found to be conversion dependent. Comparison of the experimental data on the effect of layer thickness on the maximum rate of oxygen absorption with the results of calculations (carried out assuming various overall reaction orders and applying literature data on permeability coefficient) suggests that, during degradation, the reaction rate is affected by the opposed transport processes of oxygen and the volatile decomposition products. The weight change, closely related to the transport process of volatile products, is diffusion-controlled over the whole range of thickness studied. At the same time, below a critical thickness (approx. 0·05 mm), even the maximum rate of oxygen consumption may be considered a function of only the kinetic parameters.