Using specular neutron reflectivity, we have measured the structure of tethered polymer layers immersed in a solution of mobile chains of the same chemical composition. The tethered polymer layer is a stable Langmuir film of a diblock copolymer spread on the air-liquid interface of a selective solvent. Due to the restricted conformational freedom of the grafted chain, a depletion layer at the grafting surface is expected. In our system, this depletion zone is enhanced and thus observable experimentally due to a repulsion between the polymer and the surface. The structure of the tethered layer is adequately described by a parabolic profile followed by an exponential tail, with a depletion layer at the surface. In the presence of mobile chains in solution, the grafted polymer layer is compressed: the thickness decreases and the monomer volume fraction increases. This compression may result from a re-equilibration of the osmotic pressure of the polymer layer, or from a screening of the monomer excluded volume interaction due to penetration of the mobile chains into the grafted layer. In this paper, we present our experimental data on the effects of mobile polymer concentration and chain length on the structure of the tethered chain. From these data, we construct a phase diagram to identify the regions of interpenetration of the mobile chains into the grafted layer.