The influence of the compression setting ratio on the capsule fill weigth variability has been assessed for a series of particle size fractions of Avicel, calcium carbonate, lactose, maize starch and Starch 1500, on a capsule filling simulator. For most of the systems tested, the most uniform weights were achieved when no compression was applied to the powder during the filling process. The finest particle size fractions of lactose and maize starch do however require some compression to aid powder retention. For the larger particle size fractions, providing high compression settings could induce the piston to jam in the nozzle resulting in failure to fill a capsule at all. The degree of coating of the wall of the nozzle increased with decreasing particle size and increasing compression setting. Lactose produced the greatest degree of coating of the nozzle. For nozzles pretreated by a running in process to induce nozzle coating, most powders behaved in the same manner as those which had started with a clean nozzle. The exception was lactose, which could not be effectively filled using a pretreated nozzle, due to severe binding of powder to the nozzle wall inducing jamming of the piston within the nozzle.