The biogenic fraction of polyethylene terephthalate (PET) samples produced by direct esterification of terephthalic acid and ethylene glycol with antimony trioxide catalyst was evaluated by radiocarbon accelerator mass spectrometry. At the Radiocarbon Laboratory of Universidade Federal Fluminense, we measured PET samples of different biogenic fractions and the chemical components involved in the synthesis reactions. The percent modern carbon was determined and the biogenic fractions were calculated according to Standard Test Methods for Determining the Biobased Content of Solid, Liquid, and Gaseous Samples Using Radiocarbon Analysis (ASTM-D6866). Stable isotopes were measured by conventional mass spectrometry and revealed that isotopic fractionation may be taking place in the synthesis of PET. For the theoretical 20% biobased carbon sample, the radiocarbon result lead to a 23% biogenic fraction, in agreement with 3% uncertainty anticipated by the norm. The reasons for uncertainty in the specific case of PET are discussed and seem to lie on the presence of residual ethylene glycol within the PET structure.