The internal friction results obtained by Entwistle 1,2 from a number of quench aging Al alloys are re-examined and interpreted. Two transient contributions to the internal friction are observed on aging duralumin near room temperature. The first contribution develops during the initial hardening, and is deduced to be of anelastic origin. It is suggested that this contribution occurs by a stress induced rearrangement of clusters of solute atoms, and at a rate greatly accelerated by the presence of excess vacancies. The second contribution, which develops considerably later in the aging sequence, is also a relaxation phenomenon but with a shorter relaxation time governed by an activation energy of 13.5 kcal/mole and a frequency factor of 1014 sec/t-1. This later relaxation is thought to arise from a stress induced rearrangement of small groups of solute atoms containing vacant lattice sites as an integral component of their structure. This model successfully explains the salient experimental results, and leads directly to the interpretation of the activation energy as that for vacancy motion. The observed sequence of changes indicates that the relaxation centers responsible for both contributions occur within the G.P. zones, and that a spontaneous reorganization within the zones is responsible for the changeover from one contribution to the other. © 1959.