Polymer orientation in dilute solutions undergoing shear flow is investigated by means of now birefringence using a quantity called "orientation resistance". In the framework of theoretical models it was found that the intrinsic orientation resistance is strongly influenced by polydispersity (Bossart and Ottinger, 1985). Dilute solutions of two radical polymerized polystyrene samples with different molecular weights in both good and Theta solvents are investigated by flow birefringence. The experimentally obtained values for the intrinsic orientation resistance are in very good agreement with the values calculated in the framework of kinetic polymer models. These experiments demonstrate that the polydispersity effect is well understood and that flow birefringence constitutes an extremely precise method for determining molecular weights. From our experiments it can be inferred that the number-average molecular weight M-n of a polymer sample can be measured to within a relative error of Delta M-n/M-n = 3%. The use of the birefringence method for the determination of the molecular weight distribution of a polymer sample is suggested.