Liquids are treated by ultrasound in a flow-through reaction vessel with an elongate ultrasonic horn mounted to the vessel with one end of the horn extending into the vessel interior. The liquid flow path inside the vessel is such that the entering liquid strikes the end of the horn at a direction normal to the end, then flows across the surface of the end before leaving the vessel. The end surface of the horn is positioned in close proximity to the entry port to provide a relatively high surface-to-volume ratio in the immediate vicinity of the horn end. In a further improvement, the horn is joined to an ultrasonic transducer through a booster block that provides an acoustic gain to the ultrasonic vibrations, and the booster block is plated with a reflective metal to lessen any loss of ultrasonic energy being transmitted to the horn.