This work describes the effect of solids load and ionic strength on the electrical conductivity (K-S) of concentrated aqueous suspensions of commercial alpha-alumina (1-35 vol% solids). The results obtained show that the dependency of the electrical conductivity of the suspending liquid (K-L) on the volume fraction of solids is well described by Maxwell's model. The change in the conductivity of the suspensions relative to that of the suspending liquid (K-S/K-L) was found to be inversely proportional to the solids content, as predicted by Maxwell's model. The relative conductivity rate, Delta K, could be interpreted in terms of the DLVO theory and the particles double layer parameter, kappa alpha, and used as a stability criterion. As kappa alpha changes, in response to the changes in ionic strength, so does the conducting to insulating character of the particles and, as such, their contribution to the overall suspension conductivity (expressed by Delta K). When the particles become insulating, the suspension conductivity decreases when the solids load increases. The turning point in this particle behaviour corresponds to a critical concentration of ions in the solution that destabilises the suspension and is associated with the critical coagulation concentration (ccc). It is the electrical double layer that ultimately determines the conducting or insulating character of the particles, and that character can be made to change, as required for suspension stability, and accessed by the relative conductivity rate. (c) 2005 Elsevier Inc. All rights reserved.