Solutions of KI, CCI~, and starch have been irradiated at I MHz in a focused ultrasonic field. The electrical voltage signals which appear at the transducer electrical terminals, superimposed on the drive signal, and which have previously been associated with acoustic cavitation in sonicated water, have been monitored together with the chemical changes occurring. When solutions were irradiated in a 3.3 ml container at the focal position a chemical yield was obtained in some samples irradiated at peak focal intensities of 50-388 W/cm 2, but the reproducibility of results was poor. At an intensity of 515 W/cm 2 many Willard typecavitation events occurred (i.e., microbubble clouds moving through the focus and lasting for milliseconds). They were accompanied by electro-acoustic signals of the order of volts. The chemical yield correlated with the appearance of Willard events and iodine was being produced at an average rate of 2.8 โข 10 t~ molecules per cycle of the sound wave during an event. However, when larger volumes, from 43 ml to 7-5 litres, were irradiated at intensities less than 515 x~V/cm 2, the iodine yield increased dramatically, being 6000 times larger in a 260 ml container than in a 3"3 ml one at 175 W/cm 2. The occurrence of a high yield was related to the appearance of a continuous cavitation signal of the order of 50 millivolts. The active centres could be detected as origins of streams of blue in the container. It is suggested that they are trapped in the sound field in the same manner as visible bubbles are trapped. A centre could remain active for 20 seconds compared with the millisecond lifetime of a Willard event and have a yield of about 109 molecules per cycle of the sound wave at 140 W/cm 2. The position of maximum activity is downstream from the focus and it is shown that nuclei do not need to pass through the high intensity focal region before acting as cavitation centres.