In the design of agitators for solid-liquid mixing processes there are two important criteria. The first is "just suspended" where all the particles are in motion on the base of the vessel but not necessarily uniformly distributed. The second is "uniform distribution" where the particles are distributed throughout the liquid phase so that, at each axial location within the slurry, the solids concentration is very close to the calculated average. The correlation proposed by Zwietering (1958. Chem. Eng. Sci. 8,244) has been widely used for estimating the "just suspension" speed, N-JS, and is recommended for agitator design in the Handbook of Industrial Mixing (Atiemo-Obeng et al., 2004. Solid-liquid mixing. In: Paul, Atiemo-Obeng, Kresta (Eds.), The Handbook of Industrial Mixing. John Wiley & Sons, Hoboken, NJ, p. 558). However, there is significant evidence that the correlation does not properly account for the effect of key variables, including liquid viscosity, density difference, and scale. This is likely related to the correlation's lack of physical basis and development using simple dimensional analysis, along with the fact that it was based on limited data taken over a narrow range of physical properties and scales. Another problem with the correlation is that in order to use it the value of a geometric specific constant must be known. This paper reports data taken at three vessel scales and over a wide range of liquid phase viscosities using axial and mixed flow impellers in standard baffled vessels with torispherical or dished bases. The model used to correlate the data is based on the work of Davies (1986. Chem. Eng. Proc. 20, 175). Analysis shows that there are two regimes; one where the large and medium sized particles interact with turbulent eddies in the inertial sub-range and a second where smaller particles interact with eddies approaching the viscous sub-range. The correlation developed for the inertial sub-range mechanism can be rearranged to show that the appropriate scale-up rule for the just suspension speed is constant power input per unit mass. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.