The heterogeneous polymerization of vinylidene fluoride in supercritical carbon dioxide has been investigated experimentally, and the obtained results have been interpreted through a detailed kinetic model. The comparison between model predictions and experimental data indicates the presence of two reaction loci: the continuous supercritical phase and the dispersed polymer phase. However, the presence of two reaction loci is not the result of the thermodynamic partitioning between two phases, but rather a kinetic effect. This in fact occurs because part of the radicals generated in the continuous phase, which are driven by thermodynamic equilibrium to diffuse to the dispersed phase, are actually terminated in the former before they can reach the latter, This provides a quantitative explanation for the bimodal molecular weight distributions often measured experimentally for this system.