Ultrasound-induced polymer scission is a nonrandom process which alters the molecular weight distribution of polymers. However, transient cavitation, and consequently polymer scission, is not possible in concentrated polymer solutions due to the high liquid viscosity. The addition of an antisolvent can be used to circumvent this problem because the antisolvent decreases the gyration radius of polymer chains, which induces a reduction in liquid viscosity. To determine the influence of carbon dioxide (CO2) as an antisolvent on the ultrasound-induced scission rate, ultrasonic scission experiments of poly(methyl methacrylate) have been performed in bulk methyl methacrylate (MMA) as well as in CO2-expanded MMA. Modeling the experimental time-dependent molecular weight distributions (MWD) has revealed the scission kinetics at different polymer concentrations and CO2 fractions. At low polymer concentrations, the scission rate is decreased upon an increased CO2 content. This is a result of the higher vapor pressure of CO2, which cushions the cavitation. However, at higher polymer concentrations, this effect is counteracted by the viscosity reduction induced by CO2. As a consequence, the scission rate in CO2-expanded MMA is higher as compared to bulk MMA for solutions with a high polymer concentration. The results show that ultrasound-induced scission in pressurized CO2 can alter and control the MWD of polymers even in concentrated polymer solutions, whereas ultrasound-induced scission in bulk solutions is limited to relatively low polymer concentrations.