Nanoparticles tend to form large clusters (aggregates and agglomerates) which need to be broken up when dispersing in a liquid. The dispersion of nanoparticle clusters has been studied to investigate the kinetics and mechanisms of break up with two types of particles: fumed silica and aluminium oxide. Results obtained under different processing conditions using an in-line rotor-stator are reported. It could be concluded that break up occurs predominantly through erosion in the case of silica and shattering with aluminium oxide. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.