Blending of active pharmaceutical ingredient (API) and excipients is a pre-requisite to the dry manufacture of solid dosage forms intended for oral use, whether or not granulation steps are employed prior to compaction. Excipients and API are known to be subject to tribo-electric charging, against each other and the materials in which the blends are manufactured (e.g. stainless steel 316). This study aimed to assess and compare the magnitude of tribo-electric charging of excipients and API using a material-sparing technique. Intra-sample variability in tribo-electric charging was found to be generally low. The results showed that excipients had lower charge levels and smaller variability as compared to the API materials. Some of the APIs tested charged extensively to the levels in excess of +/- 150 nC/g. It was also found that the extent of particle adhesion to surfaces of the container walls for charged API was considerably greater compared to the excipients. These results suggest that the extent and variability of tribo-electric charging of APIs is the predominant contributor to variability in electrostatic charge of pharmaceutical blends and to any related formulation issues. It is therefore reasonable to conclude that, to control the electrostatic properties of a formulation, it is a priority to control the particle properties of the API. (C) 2011 Elsevier B.V. All rights reserved.