The problem of determining the reaction locus in the supercritical carbon dioxide dispersion polymerization of methyl methacrylate is considered. For this, two limit models are comparatively evaluated using experimental data of polymerization kinetics and molecular weight distribution. The two models take opposite assumptions with respect to the relative rate of interphase radical transport with respect to the radical life time, which lead to different relative importance of the polymerization in the continuous or in the dispersed phase. The model parameters have been estimated using independent literature sources so as to ensure genuinely predictive modelling. The results clearly indicate that the interphase transport of the active chains is a key process in determining the reaction locus and it has to be carefully considered in order to reliably simulate any polymerization process of this type. (C) 2004 Elsevier Ltd. All rights reserved.