The objective of this article is to derive theoretical equations for the minimum stirrer speed in vessels containing suspensions. There are a variety of equations for the prediction of this stirrer speed (Zwietering, Chem. Eng. Sci. 8 (1958) 244-253; Kneule and Weinspach, Verfahrenstechnik 1 (12) (1967) 531-540; Nienow, Chem. Eng. Sci. 23 (1968) 1453-1459; Einenkel, VDI-Forschungsheft Nr. 595, Dusseldorf, 1979; Einenkel and Mersmann, vt-Verfahrenstechnik 11 (2) (1977) 90-94; Niesmak, Thesis TU Braunschweig, 1982; Molerus and Latzel, Chem. Eng. Sci. 42 (6) (1987) 1423-1437; Zehner, Chem. Ing. Technol. 58 (10) (1986) 830-831). However, nearly all of them are derived from experimental results obtained in a limited range of vessel size and suspension properties [1-3]. Some relationships based on experiments carried out in small vessels are contradictory and not useful for scale-up. Reliable models are necessary not only for the design of stirred vessels but also for the reduction of power consumption. It is important to distinguish between two decisive processes : the ＇avoidance of settling＇ and ＇off-bottom lifting＇. (C) 1998 Elsevier Science S.A. All rights reserved.