The changes of the average molar mass M-w, M-n, M-z, and molar mass distributions during multistep uniaxial drawing of poly[ethylene terephthalate) (PET) to achieve ultimate mechanical properties have been studied in detail by means of size exclusion chromatography (SEC) with triple detection: concentration, viscosimetry, and light scattering, using HFIP as solvent. An increase in molar mass of PET due to post-polycondensation and/or transesterfication during drawing at a high temperature of 160 to 230 degrees C was found. Moreover, drawing leads to crystallization and large orientation in the amorphous phase, which results in lower molecular mobility and prevents a further growth in chain length. Crazing under extreme drawing conditions occurs and affects a decrease in molar mass.