A comprehensive model of dispersion polymerization in supercritical media is presented. With respect to previous ones reported in the literature, this model accounts in detail for the interphase mass transport of the active polymer chains and specifically for its dependence on the chain length. All the model parameters have been evaluated from independent literature sources in the case of poly(methyl methacrylate) dispersion polymerization in supercritical carbon dioxide. The corresponding a priori predictions of the model have been validated with various sets of experimental data, covering a rather wide range of operating conditions, in terms of conversion, pressure and molecular weight distribution as a function of time. It has been found that the interphase mass transport of active chains plays a significant role in determining the number of reaction loci and consequently the final polymer properties and, in particular, the molecular weight distribution. (c) 2005 Elsevier Ltd. All rights reserved.