The addition of carboxylic acids to polyolefins results in an apparent slippage flow along rheometer and die walls. In this article, we seek to extend this investigation to polymer/particulate compounds. An experimental study on the influence of carboxylic acid additive in polyethylene/filler and polystyrene/filler compounds in rheometers is described. Capillary and cone-plate experiments are reported. A series of particle fillers of varying character, including carbon black, calcium carbonate, zinc oxide, and talc are compared. The aliphatic fatty acid, octadecanoic acid, was used as an additive. Significant viscosity reductions were observed. The greatest effects were found in polyethylene/(carbon black) and polystyrene/(calcium carbonate) compounds. All of the polyethylene compounds exhibited slip flow, but the polystyrene compounds did not. Apparently much of the octadecanoic acid was interacting with the polar particle fillers and not with the die wall. (C) 2003 Wiley Periodicals, Inc.