Experimental work was carried out in order to measure the distribution of the effective applied stress in the roll gap during roll press compaction of lactose monohydrate, alumina and sodium chloride. The purpose of the study was to determine the influence of both powder properties and roll press feeding on the compaction. Therefore, a laboratory roll press was specially instrumented. The stress applied on the compact in the roll gap was measured locally by two piezoelectric transducers. The measured stress fluctuated a lot and was not homogeneous along the roll width. This heterogeneity was characterized by measuring the transmission of light through a sodium chloride compact. In fact, the heterogeneity of the compact properties was caused by the heterogeneity of the feeding pressure. The feeding pressure heterogeneity was principally due to the powder packing that took place in the last flight of the screw feeder. The roll press compaction could be very affected by the powder packing especially when small roll presses (pharmaceutical) were used. (C) 2003 Elsevier Science B.V. All rights reserved.