The rapid evaporation or condensation of a vapor bubble when out of equilibrium compared to the slow dissolving or growth of an air bubble results in quite different propagation properties of sound waves through water containing one or the other types of bubbles. Adapting a method developed by Foldy in his treatment of the air bubble case, we derive an expression for the propagation number for sound waves in a ``vapor bubbly'' medium. As an application of the theory it is shown that as few as 10/cm3vapor bubbles of radius about 10−3cm will (for ambient pressure about one atmosphere) attenuate an ultrasonic (f=˙3·104cps) sound wave by a factor of 10 in 10 cm. Suggestions are made concerning the use of these waves for the early detection of shut‐down bubbles following a power burst in a reactor.