In situ mass-suspension polymerisation of MMA was carried out in a single reactor. The mass polymerisation was carried out in a gently agitated monomer layer of a two stratified layers of monomer and water in the reactor. The degree of conversion at which mass polymerisation changed to suspension polymerisation, by increasing the rate of agitation, was altered systematically. The polymer content of the monomer/polymer solution, formed during the mass polymerisation stage, significantly affected the evolution of the particle size distribution. Mass-suspension polymerisation was found to be more vulnerable to drop coalescence and process failure than conventional suspension polymerisation. The results indicate the importance of the transition stage in a typical suspension polymerisation during which the rate of polymerisation is very low and the adsorption of stabiliser on the surface of drops is completed. The polymer beads from the mass-suspension polymerisation had a very broad size distribution with a large contribution from satellite particles. (C) 2008 Elsevier Ltd. All rights reserved.