The propagation characteristics of 75‐MHz ultrasonic waves through molecular films of arachidic acid have been investigated. On thermal cycling through the melting temperature the ordered films break up into islands of disordered arachidic acid. With the help of an ultrasonic pulse‐difference technique we could detect changes in the transmitted signal when arachidic acid islands as thin as 500 A˚ changed from the solid to the liquid state. It is suggested that similar experiments at higher frequencies could be of value in the investigation of phase changes and general structural properties of biological membranes.