In the reported experiments, the effect of addition of various stabilizers on the thermo-oxidative stability of polyethylene terephthalate has been investigated. Thermal stability has been evaluated by the TGM- and DTA-methods. The optimum amounts of stabilizers have been determined and the stabilizing formulations proposed to protect PET under processing conditions. The following stabilizers gave good results: Stabaxol P, Ipognox 66, Irganox 1010 in compositions with Epidian 6 resin or polyamides.