The ultrasonic absorption of large unilamellar vesicles (average diameter 0.2 μm) was determined in the frequency range 0.5–5 MHz. The liposomes were composed of a 4:1 mixture by weight of dipalmitoyl phosphatidylcholine and dipalmitoyl phosphatidylglycerol. A large increase in absorption occurs at the solid to liquid-crystalline phase transition temperature (42 °C) of the pure lipid vesicles. This increase in absorption is interpreted as a structural relaxation of the “melting” fatty acid chains occurring with an average relaxation time of 76 ns. The liposomes were also found to be extremely permeable near the transition temperature, releasing encapsulated cytosine arabinoside. Addition of cholesterol or gramicidin to the liposome bilayer broadened the ultrasonic absorption and reduced the efflux of cytosine arabinoside at the transition. Gramicidin, in addition to broadening the transition, slows the isomerization of bonds in the hydrocarbon chains of the lipids. A concentration of 5 mole percent gramicidin increased the average relaxation time to 210 ns. [Work supported in part by grants from the National Cancer Institute, National Institutes of Health, and the American Cancer Society.]