From rheology it is known that entangled polymer melts or solutions do not obey the stick boundary condition at a solid wall when exposed to high shear stress. This may display an interfacial stick-slip transition (SST) where the apparent flow rate jumps discontinuously due to a massive wall slip. Although this phenomenon is frequently observed in polymer tribology and several theoretical models exist to describe it, a direct experimental observation of its origin is still lacking. In our experiment we will use in situ shear neutron reflectometry to record the theoretically proposed structural change during SST.