Incineration or burning of plastics involves the complicated interaction of thermal degradation, pyrolysis, combustion of pyrolysis products, mass and energy transport. Essential steps in the over-all process are the thermal degradation and formation of gaseous products that determine amongst others the effective rate of combustion. This paper reports some new results for the kinetics and mechanisms of the thermal decomposition of PVC and polyamide 6. The over-all reaction rate for thermal decomposition is measured applying thermo-gravimetry (TGA) with simultaneous analysis of the evolved gases by mass spectrometry. The kinetic parameters are determined by fitting a kinetic model to the measured decomposition rates with the help of a direct search method. For PVC different mechanisms during the dehydrochlorination with different reaction rates may occur depending on the sample size for TGA. For polyamide 6 a single mechanism for decomposition is found.