An experimental study of the influence of carboxylic acid additive in polyethylene/filler and polystyrene/filler compounds in slit die is described. Slit die extrusion experiments and contact angle measurements are reported. A series of fillers of varying particle size including carbon black, calcium carbonate, zinc oxide, and talc are compared. Aliphatic fatty acids, especially octadecanoic acid were used as additives. The greatest viscosity reduction was found in polyethylenel(carbon black) and polystyrene/(calcium carbonate) compounds. Smaller effects are found with the other polyethylene/filler compounds and none in polystyrene/filler compounds. Contact angle experiments indicated carboxylic acid layers on the die surfaces for the polyethylene and not the polystyrene compounds. Mechanisms for this behavior are discussed.