Abstract The acceleration of chemical reactions due to an ultrasonic field is the basis of a number of methods for the visualization of this field. Such methods have been examined with qualitative and quantitative experiments to determine the mechanism of ultrasonic action. It is found that the standing wave created by reflection of the ultrasound acts upon a diluted substance. The substance is concentrated at the displacement nodes of the standing wave and consequently in these regions the reaction is accelerated. An explanation is also given for some previous experiments.