A series of maxima in the internal friction of aluminum at temperatures between 4.2° and 270°K are reported and analyzed. These maxima follow cold work and fast cooling of the specimen and occur on warm‐up. The maxima are related to a mechanism of vacancy pinning of dislocations. Vacancies are indicated by the annealing characteristics of the phenomena. The vacancies in the model are supposedly produced by the cold work and are associated in strings or clusters. Extended dislocations move over saddle interaction energies and produce the relaxation maxima found at kilocycle frequencies. The megacycle attenuation measurements indicate the temperature at which the dislocation breaks from the pinned configuration.