The thermal degradation of poly(methyl methacrylate) (PMMA) and poly(ethylene terephthalate) (PET) has been studied by non-isothermal thermogravimetry and a comparison made with isothermal studies. Several analytical methods have been applied to the analysis of the non-isothermal thermogravimetric (TG) data to elucidate the validity of the procedures adopted and to compare the results with isothermal procedures. In addition an alternative method of analysing the non-isothermal data is proposed, which addresses some of the shortcomings associated with the previous methods. It was found that thermal lag has a significant impact upon the calculated kinetic parameters, and extrapolation of the kinetic parameters to zero heating rate was required. It was also found that structural changes in the polymer residue during degradation brought about changes in the observed reaction order, and the kinetics of thermal degradation could only be studied to low fractional conversion.