The effect of energy input, pH and temperature on de-aggregation of hydrophilic silicon dioxide powder (particle size 12 nm) in a high shear mixer was investigated. It has been found that de-aggregation is a two step process. Initially, at low energy input very large aggregates (3-1000 mu m) are gradually broken into smaller secondary aggregates (2-100 mu m) of a single modal size distributions. As the energy input increases primary aggregates (0.03-1 mu m) are eroded from the secondary aggregates leading to bimodal size distributions with the first mode between 0.03 mu m and 1 mu m corresponding to the primary aggregates and the second mode between 2 mu m and 100 mu m corresponding to the secondary aggregates. At a sufficiently high energy density all secondary aggregates are broken into primary aggregates however, even at the highest energy density employed the primary aggregates could not be broken into single nano-particles. The temperature and the pH affect deaggregation kinetics but do not alter de-aggregation pattern. Increasing pH at low temperature speeds up de-aggregation, whilst increasing pH at high temperature slows down de-aggregation process. (c) 2007 Elsevier B.V. All rights reserved.