In this study, three mass spectrometry techniques matrix-assisted laser desorption ionization mass spectrometry (MALDI-TOF MS) combined with thin-layer chromatography (TLC), laser desorption/ionization on silicon-mass spectrometry (DIOS-MS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) were used to investigate the thermooxidative degradation and resultant discoloration of poly(ethylene terephthalate) PET that frequently occurs during melt processing. The MALDI-TOF MS spectra of the degraded polymer showed the formation of oligomers containing carboxyl end groups via chain scission at the ether link present in PET. In addition, a variety of cyclic oligomers are found to form via two different mechanisms from the linear oligomers of the virgin PET. Thus, our results not only confirm the oligomers containing carboxyl end groups and cyclic oligomers as the low molecular weight degradation products but also deepen the understanding about their structures, molecular weights, and molecular weight distributions. Furthermore, a MALDI-TOF MS study directly on the TLC plate and DIOS-MS enable us to identify low molecular weight compounds (not detected previously due to matrix interference) that are contributors to color formation.