Concept of the transfer lag was introduced to a basic form of the Arrhenius-type rate equation to interpret the results of plastics pyrolysis in a thermogravimetric analyzer. A first-order system with time constant τwas assumed between ambient temperature and sample average temperature changes and τwas varied with the degradation of the sample. This type of model successfully described most features of plastics pyrolysis with various heating methods of the thermogravimetric analyzer. But the model was still incomplete to express the degradations when the secondary reactions were important, such as those in the latter phase of pyrolysis and those at lower temperatures.