Departure from Newtonian behavior of molten poly(ethylene terephthalate) (PET) flowing at steady state in a capillary is shown to occur at a constant shear stress of∼9.65×105 dynes/cm2.However, the shear rate for departure of PET from Newtonian behavior [γ̇w(departure)] is a function of temperature and molecular weight. An equation forγ̇w(departure) as a function of temperature and molecular weight is derived;γ̇w(departure) increases as melt temperature increases for a given molecular weight and decreases as molecular weight increases for a given temperature. Maximum relaxation times for viscoelastic response of molten PET can be approximated from the reciprocal ofγ̇w(departure), at least for the ranges of molecular weight and temperature investigated.