The glidant properties of compacted hydrophilic and hydrophobic colloidal silicon dioxides were compared with respect to mixing time and mixer type using microcrystalline cellulose, pregelatinized starch and a-lactose-monohydrate as model excipients. Flowability studies, including angle of repose measurements and a novel dynamic conveyor belt method, showed differences in the flow enhancement between the colloidal silicon dioxide types. An influence of mixing conditions on flowability was also observed for hydrophilic colloidal silicon dioxide. These differences could be explained by SEM and XPS investigations, indicating the importance of a uniform coverage of the glidant particles on the excipient's surface. The influence of size and distribution of the colloidal silicon dioxide particles on the surface of the excipient, mixing time, mixer type are described in detail. In addition, moisture studies showed that colloidal silicon dioxide protects the excipients against a flowability decrease caused by humidity. (C) 2004 Elsevier B.V. All rights reserved.