Polymer-surface interactions in flowing systems were studied using an attenuated total reflectance Fourier transform infrared spectroscopy technique. The absorbance of deuterated polyethylene, d-PE66 (M-w = 6.60 x 10(4)) and d-PE112 (M-w = 1.12 x 10(5)), near the surface of a flat zinc selenide crystal was followed as it was replaced by linear low-density polyethylene (Mw = 1.07 x 10(5)). The experiments were performed under wall stresses of 0.090, 0.134, and 0.155 MPa using a rectangular flow channel formed by a ZnSe crystal and an aluminum block. The decay profiles depended on the molecular weight distribution, the chemical composition distribution, and the shear stress. For d-PE112, the decay profiles suggest that slip is due to a cohesive failure. For d-PE66, the slip mechanism appeared to be due to adhesive failure. For d-PE112 with larger and regular chains, an immobile layer forms whose thickness (12-17 nm) depended on the wall shear stress.