Résumé : The acoustic signal from the sonochemical production of H2O2in water, as measured by the intensity and the width of the second harmonic, show a sensitive and correlated dependence to the presence of small amounts (millimolar range) of an anionic surfactant (SDS) in water. The graphic shows the link from the ultrasonic reaction to the measurable quantities. New possibilities to reliably control such processes is therefore opened.