The thermal relaxation of crystalline and amorphous polyethylene terephthalate, PET, i.e. the enthalpic relaxation or physical ageing, has been measured using a range of thermal analytical techniques, namely DSC, DMTA, and DETA. While the relaxation is associated with the amorphous regions, in general, these are constrained by the presence of crystalline regions out of proportion to the amounts present, such that the glass transition temperature T-g is raised to higher values and the transition itself broadened. The change in specific heat associated with the transition is also reduced and measurement of the T-g of partially crystalline material by DSC becomes difficult. Chain mobility is greatly reduced and this has a marked effect in reducing the rate of enthalpic relaxation but not eliminating it entirely.