Emulsion‐type polymerization reactions have been accelerated by irradiation with ultrasonic energy.A magnetostriction oscillator at 15 kc gave essentially the same results as a piezoelectric oscillator at 500 kc. The rate of formation of polystyrene was doubled and higher final yields obtained using about 0.03 watt of vibrational power per cc of latex. Experiments were made to separate local heating effects from those due to vibrational energy.A thermostatted stainless steel cell was developed for irradiating butadiene emulsions under pressures up to 60 p.s.i. About ½ watt of vibrational power per cc of latex doubled the rate of reaction for GR‐S, Redox, and Redsol formulas.The ultrasonic intensity apparently must exceed a critical value before any appreciable accelerative effect is observed. Above the critical intensity, the time to reach a given yield of polymer is approximately inversely proportional to the power used.