The development of a new reaction medium is reported, based on a biphasic mixture of carbon dioxide and water for emulsion polymerizations:  a surfactant-free aqueous emulsion polymerization of methyl methacrylate using potassium persulfate under a varying head pressure of CO2 (0−350 bar) at 75 °C. The resulting polymer is a stable latex with particles of submicron size. The effect of CO2 on polymerization is relatively small, until there is a significant change at high CO2 pressure. This is seen by examining the molecular weight distribution in the form of the log(number distribution), P(M) (readily obtained by GPC). At pressures of 140 bar and below, the P(M) show the form expected for chain-stopping events dominated by transfer and by diffusion-controlled termination. At 280 bar, ln P(M) is significantly steeper than its lower-pressure counterpart at relatively low conversion (45%). This is attributed to swelling by supercritical CO2 reducing the viscosity of the particles, allowing more rapid termination.