Free radical copolymerization reactions of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) were carried out in supercritical carbon dioxide at T = 50 degrees C. When ammonium carboxylate perfluoropolyether surfactants were used, the formation of polymer particles was observed provided that HFP mole fraction in the feed was not larger than 0.30. Under these conditions, the effect of the total interphase area of the polymer phase, A(p), on the product properties was investigated by comparing experiments with largely different amounts of stabilizer, i.e., ranging from precipitation (smaller A(p)) to dispersion (larger A(p)) polymerization systems. We found that the copolymer molecular weight distribution (MWD) is heavily affected by A(p). This experimental evidence strongly supports the presence of two reaction sites, the CO2-rich and the polymer-rich phase: the contribution of both is important in the examined range of operating conditions.