Using neutron reflectivity, we found that there is no intrinsic depletion layer at a deuterated polystyrene (dPS) film and deuterium oxide (D2O) interface. A spun-cast film is susceptible to contamination on its surface from its surroundings during sample preparation. A contamination layer of hydrogenated organic material will be detected as a reduced scattering length density layer at the interface. We demonstrate that, by careful treatment of the film, contamination would be the primary cause of the reduced scattering length density layer at the interface.