Simultaneous measurements of stress and birefringence have been performed during uniaxial extension at constant strain rate of a polydisperse polystyrene melt. In contrast to melts with narrow molecular weight distribution, the relationship between stress and birefringence is found to depend on the temperature and the strain rate in the non-linear region. Failure of the stress-optical rule initiates for lower stress levels, the actual critical stress increasing with the reduced extension rate towards the value measured for quasi-monodisperse systems. In agreement with predictions by van Meerveld [J. van Meerveld, Validity of the linear stress optical rule in mono-, bi- and polydisperse systems of linear entangled chains, J. Non-Newtonian Fluid Mech. 123 (2004) 259-267], this premature failure of SOR occurs within the strain rate regime where only a fraction of the chains of the melt is expected to stretch significantly. (c) 2006 Elsevier B.V. All rights reserved.