It has been shown that ultrasonic irradiation can induce the formation of free radicals in liquid carbon dioxide. In this work, ultrasonically induced polymerization of MMA or styrene in liquid CO2 was successfully performed. With ultrasound irradiation for 2–6 h and the absence of any form of stabilizer, discrete polymer particles have been generated and the resulting polymer formed as a fine powder. The results also show that the irradiation period and CO2:MMA ratio have a significant effect on both the molecular weight and molecular weight distribution for the resulting polymer.