The strongly varying local flow conditions in agitated vessels influence the mean diameter and (fie particle size distribution during the suspension polymerisation process. For the suspension polymerisation of styrene with a dispersed phase content of 50%, the analysis of the particle size distribution gives bi- or trimodal curves with only three characteristic shapes, This is valid for different recipes. mixing conditions, and reactor sizes. An explanation for this phenomenon is given by an analysis of the local flow conditions in connection with the motion of the particles. Using these results yields models for the mean particle size and the possible particle size distribution. these models include well-known laws for the dispersion process and estimate the range of validity. Therefore. it is possible to give an explanation for the variety of partly contradictory empirical equations in the literature. (c) 2005 Elsevier B.V. All rights reserved.