FT-IR spectroscopy has been used to follow changes in chain conformation of poly(ethylene terephthalate) (PET) which occur during glass formation and physical ageing. It has been shown that there is an increase in the population of gauche ethylene glycol conformation at the expense of the trans on cooling the liquid and on physical ageing. This is contrary to a previous interpretation and it is implied that the trans has the higher energy of the two conformations since it is considered to produce less well packing of the polymer chains.