The influence of the molecular weight distribution (MWD) of bidisperse entangled linear polymer systems on the critical stress at which the stress-optical rule (SOR) fails for uniaxial extensional flows is investigated numerically. The model predictions show that the critical extensional stress where the SOR fails, Deltatau (FSOR), depends on the molecular weight distribution and the extension rate. The variation in Deltatau(FSOR) results primarily from the change in the mass fraction of stretched chains. In addition, the interplay between the contribution from the non-stretched and stretched chains to the stress and birefringence is significant for the magnitude of Deltatau(FSOR). The latter is the reason why Deltatau(FSOR) is larger compared to that expected from the mass fraction of stretched chains alone. A comparison of the model predictions with experimental data for a nearly monodisperse concentrated polystyrene (PS) solution, reported in the literature, shows that the SOR fails for a chain stretch of about one-third of the maximum stretch ratio. For bidisperse PS melts, the critical stretch ratio varies with the MWD. For monodiperse PS melts, the critical stretch ratio is about two-fifth of the maximum value. A simplified procedure is proposed to analyze polydisperse melts. Compared to experimental data reported in the literature, the trend is qualitatively correct, but the predicted magnitude of Deltatau(FSOR) is too low. (C) 2004 Elsevier B.V. All rights reserved.