In the previous work, we have studied the effects of acoustic intensity, argon gas flow rate, surfactant concentration, and initial monomer concentration on polymerization rate, polymer particle number, and polymer molecular weight in the ultrasonically initiated emulsion polymerization of methyl methacrylate. In this study, radical trapping experiments were used to investigate the effect of acoustic intensity, argon gas flow rate, and sodium lauryl sulfate concentration on the extent of free radical generation in aqueous sodium dodecyl sulfate solutions. In these radical trapping experiments, aqueous solutions of sodium lauryl sulfate were ultrasonically irradiated in the presence of a radical scavenger. The sodium lauryl sulfate molecule degraded under ultrasound to form free radicals in the water. It was found that the extent of free radical generation increased as: (1) the 0.60 power of the acoustic intensity, (2) the 0.44 power of the argon gas flow rate, (3a) the 0.35 power of the surfactant concentration within the 0.035M-0.139M surfactant concentration range, and (3b) the 1.09 power of the surfactant concentration within the 0.139M-0.243M surfactant concentration range. The generated free radical concentration, the number of polymer particles produced, and the polymerization rate exhibit an increasing trend with an increasing (1) acoustic intensity, (2) argon gas flow rate, and (3) surfactant concentration. Increases in surfactant concentration correspond to: (1) an increase in generated free radical concentration and (2) a decrease in polymer molecular weight. These relationships confirm the previous assumptions that (1) the polymerization rate increases with increasing acoustic intensity and argon gas flow rate due to an increase in the radical generation rate and the reaction temperature; (2) the polymerization rate increases with increasing surfactant concentration due to an increase in the radical generation rate, the micellar, homogeneous nucleation, and the reaction temperature; and (3) the polymer molecular weight decreases with increasing surfactant concentration due to an increase in the radical generation rate. These results are helpful in understanding the kinetics of the ultrasonically initiated emulsion polymerization of methyl methacrylate.