Molecular relaxation curves of five different polystyrene samples, four monodisperse of weight-average molecular weight ranging from 210 000 to 2 340 000, and one polydisperse, have been studied using birefringence and polarization modulation infrared linear dichroism, during and after a step-strain uniaxial deformation at temperatures between T-g and T-g + 60 degrees C. Relaxation measurements can be fitted with a set of exponential decay functions, thus defining three different relaxation times. The relaxation times thus obtained with birefringence and polarization modulation experiments are similar, and decrease with an increase in temperature. The first relaxation time (tau(1)), which is of the order of seconds, is independent of average molecular weight (M-w), while the second (tau(2)) and third (tau(3)) relaxation times increase with molecular weight. For example, at T-g + 20 degrees C, and for a PS molecular weight of 2 340 000, Values of 0.7, 39 and 16 500 s were determined for tau(1),tau(2) and tau(3), respectively. The power Iatv dependence found for the third relaxation time scales as M-w(1.6) at each temperature. This behavior is consistent with the prediction of the theoretical model of Doi-Edwards and allows the assignment of tau(1) to the first, and tau(3) to the second relaxation time of Doi-Edwards (tau(2) is intermediate between those two). The decrease of the relaxation time ratio (tau(3)/tau(1)) With temperature, noted for ail the PS molecular weights investigated, suggests a decrease in the number of entanglement points with an increase in temperature above T-g+ 20 degrees C. Finally, for a given temperature, a linear dependence is observed between tau(3) and tau(1) in agreement with the tau(3) = 2 tau(1)N(1.6) Doi-Edwards prediction, where N is the number of entanglements.