This paper deals with the micromixing phenomenon occurring when ultrasonic waves propagate in a reacting medium. Parallel competing reactions A + B --> R and C + vB --> S were employed to quantify the micromixing. We have used a 20 kHz horn in a batch reactor to investigate the influence of the acoustic power. Various vessels have been tested in order to study the influence of the reacting volume (100-500-3750 ml). The dissipated power, and not the power per unit mass was found to be the determining variable. Then a classic impeller was compared to the ultrasound device in aqueous media. Values of characteristic micromixing time around 0.005 s have been obtained for both the mixing processes. But if ultrasound can generate micromixing, their energetic performances are still far lower than Rushton turbine ones. Thus the ultrasound technique must be used in a small zone to generate locally a mixing on a small scale. Finally, the ultrasound technique was also tested in more visquous liquids (2-170 mPa s) and proved to be of great interest.