Although process development is often done in well agitated vessels (e.g. with a retreat curve, pitched blade turbine etc.), there are a sizeable number of contract manufacturers' still deploying a significant number of anchor agitated process units. For the purpose of observing the Zwietering constant value 'S' and few industrially important solid-liquid systems, we conducted extensive suspension experiments with anchor agitated vessels for varying D/T ratios (0.74 and 0.73). In this study, Zwietering's N-js (just suspension speed) and the corresponding 'S' factor were obtained for each system over a range of solid loadings. We found that the Zwietering constant was strongly dependent on the nature of the solid-liquid system; i.e. different systems had different 'S' values for the same geometrical configuration. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.