Free radical polymerization of ethylene in an intermediate pressure and temperature range (P(ethylene) < 250 bar and 50 °C < T < 90 °C) in the presence of an organic solvent has been studied. Under selected conditions (P, T) and according to the amount of organic solvent added, either a supercritical monophasic or a biphasic medium is obtained. In the case of a biphasic medium, polymerization occurred in the liquid phase in which radical initiator and ethylenes are dissolved. The transition between a monophasic to a biphasic medium has been predicted using thermodynamic calculations and has been related to experimental observations such as the dependence of polymerization activity versus solvent volume.