X ABSTRACT: A polystyrene melt, whose molecular weight (weight-average molecular weight ) 1050) is about that estimated for a Rouse segment, has been studied with depolarized photon-correlation and viscosity measurements. Theoretically the dynamic depolarized light scattering is shown to be affected by the reorientational motion of the whole polymer molecule in the long-time region and the local segmental motions in the short-time region. The elastic dumbbell model is used to describe the viscoelasticity and reorientational motion of the polymer molecule. Relaxation times 〈U〉 and Uv are extracted from the depolarized light scattering and viscosity measurement results, respectively. The ratio Uv/〈U〉, being independent of temperature as expected, is of the order of magnitude predicted from the theoretical analysis. We come to a similar conclusion supporting the above results by analyzing the depolarized photon-correlation results and the viscoelastic relaxation data in the glass-rubber transition zone of entangled polystyrene melts, which are available in literature.