The streaming birefringence of dilute solutions of polystyrene in Aroclor is time dependent if the shear stress expressed as parameter β=M(η−η 0 )G/cRT is high enough. The stress‐optical coefficient Δn sin 2χ/2G (η−η 0 ) is time independent, while the quantity C=Δn cos 2χ/Gβ (η−η 0 ) decreases markedly with time of shearing. The phenomenon is apparently connected with formation of aggregates during flow. Inside the aggregate the molecules are entangled which results in a higher number of effective chains and a lower effective molecular weight. The apparent decrease of C is believed to be the consequence of the use of the molecular weight of the single molecule instead of the smaller effective M of the chain section between two entanglements. The decrease of C starts rapidly with higher concentration and velocity gradient while with lower concentration and gradient one has a pronounced induction period. The characteristic time τ for the later part of decrease is inversely proportional to the gradient. After stopping the rotor the solution recuperates slowly; the characteristic time τ′ of the reverse process is markedly dependent on the molecular weight of polymer and on solventviscosity. Higher concentration and gradient lead to higher collision rate and hence to an early aggregate formation. This yields a rapid decrease on the effective M and hence C in contrast to the situation at lower concentration and gradient. In the subsequent region of the time dependence which is independent of concentration, the aggregates proceed with internal entanglement toward their equilibrium structure. The reverse process is the disintegration of aggregates in solution at rest which is governed by molecular weight of polymer and solventviscosity.