A method and apparatus are provided for the controlled application of ultrasonic energy for conditioning of mixtures of gas and liquids by evolving and/or agglomerating gas bubbles existing with or in a liquid or for coalescing droplets of liquid dispersed in another liquid. The invention in preferred embodiments thereof comprises a coalescing apparatus for increasing the droplet size of a mixture formed as a liquid dispersed in another liquid, and a de-gassing apparatus arranged to evolve and/or agglomerate gaseous bubbles in a gas/liquid mixture. In the apparatuses, ultrasonic transducers are used to impart vibrational energy to the mixtures.