An experimental study of slippage induced in various thermoplastics in rheometers by the presence of small amount of low molecular weight additives is described. Capillary and cone-plate experiments are reported. A series of polymers of varying polarity including polyethylene, polypropylene, polystyrene, poly (methyl methacrylate) and polyamide 12 are compared. Five eighteen-carbon amphiphilic omega-compounds are used as additives including octadecanoic acid, octadecanamide, octadecanol, calcium stearate and zinc stearate. The greatest effects were found in polyethylene and polypropylene. Little or no effects are found with the other polymers. The most effective additives were octadecanoic acid and zinc stearate. Octadecanamide and octadecanol had some effectiveness. Mechanisms for this behavior are described.