Using attenuated total-reflectance infrared spectroscopy (ATR/FTIR), the concentration of deuterated polybutadiene near the surface of a flat zinc selenide crystal was followed as it was replaced by ordinary polybutadiene by flow and diffusion. Experiments were performed in the melt, both below (M similar to 1,500) and above (M similar to 15,000) the entanglement threshold. The decay profiles agree well with a finite-element simulation of the system. In contrast to previous investigations of C-16＇s, the decay profiles of both unentangled and entangled polybutadienes are consistent with a uniform diffusivity in the near-wall region.