Time-resolved rheometry was employed to study thermal and thermo-oxidative degradation of poly(ethylene terephthalate) (PET). Degradation results in a vertical downward shift of the complex viscosity in air atmosphere. We conclude that reduction of moduli and viscosity is governed by oligomers emerging from chain scission and acting as plasticizer. Additionally, cross-linking leading to a yield stress is observed at long times. In nitrogen atmosphere, polycondensation increases the molar mass and viscosity and extends the shear thinning regime. With longer degradation times, thermal degradation prevails and leads to a vertical downward shift. The reaction kinetics of three PETs with different molar mass was analyzed by a time constant tau assuming first-order kinetics. The low molecular weight PET exhibits the largest time constant in air atmosphere, and hence the lowest degradation rate, while high molecular weight PET exhibits a small tau and rapid degradation. The enhancement rate in nitrogen is vice versa.