The formation of a lubricant layer at the polymer-metal die interface, when a blend of high density polyethylene (HDPE) and a fluoropolymer processing aid (Dynamar) was extruded through a slit die, was verified and investigated in depth. Blends containing 0.5 and 5 wt% of Dynamar were prepared by a twin-counter-rotating-screw extruder. The viscosity of the blends was measured with a capillary rheometer. A specially made slit die block containing two pieces of insert was used. The inserts which could be taken out from the slit die block at the end of the measurement were separated from the polymer block by acid etching. The polymer block contains the information about the lubricant layer formed between the extrudate and the die wall. X-ray photoelectron spectroscopic (XPS), secondary ion mass spectrometric (SIMS), and scanning electron microscopic (SEM) analyses of the polymer blocks confirmed the presence of a Dynamar-rich lubricant layer at the interface between the extrudate and the die-wall surface. The thickness of the lubricant layer varied from 5 to 15 mu m depending on the shear rate and the concentration of the Dynamar in the blend. For the Dynamar concentrations and shear rates considered in this study, the efficiency of the viscosity reduction appears to be independent of the lubricant layer thickness.