A screen heater with a gas sweep was developed and applied to study the pyrolysis kinetics of low density polyethene (LDPE) and polypropene (PP) at temperatures ranging from 450 to 530 C. The aim of this study was to examine the applicability of screen heaters to measure these kinetics. On-line measurement of the rate of volatiles formation using a hydrocarbon analyzer was applied to enable the determination of the conversion rate over the entire conversion range on the basis of a single experiment. Another important feature of the screen heater used in this study is the possibility to measure pyrolysis kinetics under nearly isothermal conditions. The influence of the mixing process in the gas phase on the measured hydrocarbon concentration versus time curve was assessed and it was demonstrated that the residence time distribution of the gas phase, which has to be accounted for to correctly interpret the experiments, becomes the limiting factor when measuring pyrolysis kinetics at high temperatures and not the heat transfer rate. With this type of apparatus, pyrolysis reactions with a first order rate constant lower than 2 s-1 can be studied, which implies that the pyrolysis kinetics of the forementioned polymers could be determined at temperatures below 530 C. The kinetic constants for LDPE and PP pyrolysis were determined, using a first order model to describe the conversion rate in the 70-90% conversion range and the random chain dissociation model for the entire conversion range. Our experiments revealed that both LDPE and PP posses the same conversion rate, which is unexpected behavior since PP should be more sensitive to thermal degradation than LDPE. A comparison of the thermo gravimetric analyzer results with those obtained with the screen heater indicates an enhancement of the pyrolysis kinetics in the latter equipment. Several hypothesis were tested to explain this phenomenon and led to the suspicion that the discrepancy is possibly due to the effect of the electrical current passing through the screen on the pyrolysis reaction, although most of the evidence for this hypothesis is indirect. Screen heaters can therefore not be used in this configuration to measure the pyrolysis kinetics, if this hypothesis is correct. In addition to the experimental work two single particle models have been developed which both incorporate a mass and a (coupled) enthalpy balance, which were used to assess the influence of internal and external heat transfer processes on the pyrolysis process. The first model assumes a variable density and constant volume during the pyrolysis process, whereas the second model assumes a constant density and a variable volume. An important feature of these models is that they can accommodate kinetic models for which no analytical representation of the pyrolysis kinetics is available. Model calculations revealed that heat transfer limitations were not important during the pyrolysis experiments performed in the screen heater and could not explain the forementioned results.