This paper presents a technique which enables on-line monitoring of activated sludge flocculation. The usefulness of this technique is demonstrated by investigating the effect of shear on steady-state floc sizes. The technique is based on a method developed to investigate flocculation of inorganic particles. Samples of activated sludge are sonicated for 3 min at 50 W to produce the necessary primary particles to observe flocculation. The samples are then stirred in a batch mixing vessel and sized on-line using a Malvern Mastersizer/E. The dynamics of flocculation were found to be reproducible and follow the principles of inorganic shear induced flocculation. The median flee size was found to increase until an equilibrium between the rates of aggregation and breakage was reached. At this point, a steady-state floc size was maintained. The effect of shear on the steady-state flee size was quantified using the experimental technique. The change in floc size-with shear followed a power law relationship. This relationship was used to investigate the breakage mechanisms. (C) 2000 Elsevier Science Ltd. All rights reserved.