Fourier transform infra-red spectroscopy coupled with photoacoustic detection (PA FTi.r.) has proved to be a useful tool for finding out about quantitative structural changes in solid materials. Poly(ethylene terephthalate) (PET) is a well known semicrystalline polymer that shows important changes on annealing. In order to obtain a complete picture of isomer distributions in industrial PET samples, spectroscopic measurements were correlated with differential scanning calorimetry (d.s.c.). The results obtained indicate that the structural characteristics of the thermally treated samples are related to the fabrication process. Two different strata in the plates can be distinguished : a skin layer and the core. The correlation between the apparent degree of crystallinity of the surface obtained by d.s.c. and the percentage of trans isomer obtained by PA FTi.r. allows the latter parameter to be separated into crystalline and amorphous trans isomer and to follow its evolution with the annealing process. Amorphous trans isomer vanishes at the primary isomerization (approximately 100-degrees-C) while crystalline trans and gauche conformations show sigmoidal evolution. At higher annealing temperatures (> 140-degrees-C) the ordered trans conformation shows an approximately linear increase at the expense of the gauche conformation. Finally, a direct correlation between the ordered trans isomer and the apparent degree of surface crystallinity can be made.