The precipitation-hardening process in aluminum alloy 2219 has been investigated by means of dynamic measurements of sound wave velocity, ultrasonic attenuation and hardness. Measurements of these properties as a function of aging time at constant temperatures (150, 175 and 220 °C) were found to exhibit prominent changes and anomalies that were related to the formation of θ″ and θ′ precipitates in aluminum alloy 2219. From the temperature dependence of the sound velocity and ultrasonic attenuation, the activation energies for formation of θ″ and θ′ were found to be 4.7 kcal mol −1 and 9.9 kcal mol −1 respectively. A thermally activated process, with an activation energy of 26.6 kcal mol −1, was apparently responsible for the loss of coherency of θ′ precipitates and the resulting decrease in hardness. The occurrence of contemporaneous peaks in both hardness and ultrasonic attenuation provides the experimental evidence for this assumption. The growth law for θ′ particles was determined from the precipitation kinetics. The θ′ precipitates, in their semicoherent form, follow a two-dimensional growth law. The coarsening of θ′ and the subsequent loss of coherency are approximately governed by a three-dimensional growth law. Sound wave velocity measurements indicate that the ultimate value of the elasticity of aluminum alloy 2219 depends on the age-hardening temperature. The elastic moduli increase with increasing aging temperature. This behavior is attributed to the relatively larger volume fraction of precipitates, of intrinsically higher elastic moduli than the matrix, that results from aging at increased temperatures. The plastic behavior, however, as manifested by peak values of the hardness, was found to decrease with increasing aging temperature. The frequency dependence of the ultrasonic attenuation indicates that a relaxational mechanism is operative. The fact that the occurrence of attenuation peaks depends also on the average distribution of the precipitate size is evidenced by the effect of plastic deformation prior to aging on the location of the attenuation peaks. The present investigation has demonstrated the operational feasibility of an ultrasonic non-destructive evaluation method for monitoring the precipitation process, over a wide temperature interval, in aluminum alloys during its progress.