Ultrasonic relaxation spectra of dipalmitoyl lecithin vesicles have been recorded as a function of temperature over the frequency range 14-265 MHz. A relaxation process is observed with a time constant of about 10(-8) sec. At the mid-point of the crystalline-liquid crystalline transition (about 41.3 degrees), the relaxation amplitude is maximal. This suggests that the relaxation process is intimately associated with the order-disorder transition. Further support for this conclusion comes from the finding that the volume change of the reaction, as calculated from the relaxation amplitude at the transition midpoint, agrees with that determined independently by equilibrium dilatometry measurements of the deltaV of the transition. The results show that a major step in the transition occurs on a far shorter time scale than previously recognized. Similar fast processes have also been detected in dimyristoyl and distearoyl lecithin vesicles. From a consideration of various lines of evidence, it appears that the relaxation monitors the elementary step associated with the isomerization of lipid chains, such as kink formation through internal bond rotations, as the bilayer transforms between ordered and disordered phases.