In the ultrasonically initiated emulsion polymerization of PMMA (1), the surfactant degrades under the ultrasonic irradiation and serves as the initiator to provide the radical for the initiation process. The concentration of the primary radical produced from surfactant depends on three parameters: 1) acoustic intensity, 2) surfactant concentration, 3) argon gas flow rate. The relationship between the effect of these three parameters on the primary radical concentration, and the relative polymerization rate is similar. This similarity is useful in understanding the kinetics of this polymerization system.