Gold(III) (tetrachloroaurate(III)) was reduced in an aqueous solution containing only a small amount of 2-propanol to form colloidal gold nanoparticles in a standing wave system generated by a 200 kHz ultrasonic generator. The rates of gold(III) reduction and the sizes of the formed gold particles could be sonochemically controlled by controlling the irradiation parameters such as the temperature of the solution, the intensity of the ultrasound, and the positioning of the reactor. The size of gold particles strongly depended on the rate of gold(III) reduction, suggesting that the rate of gold(III) reduction affects the initial nucleation of the gold particles.