The precipitation at the grain boundaries during aging at 200℃ of an Al-4% Cu alloy was studied by measurements of the internal friction associated with the viscous behavior of grain boundaries. This internal friction was measured also at 200℃ with a torsion pendulum having a frequency of about one cycle per second. The results showed that the internal friction first decreases with aging but starts to rise after an aging of 6 1/2 hours. At very long aging times, the internal friction decreases again as the aging proceeds. Such a change of internal friction indicates a corresponding change in the nature of the "precipitates" at the grain boundaries during aging. This is in conformity with the results of metallographic examinations. A preliminary understanding toward the nature of these "precipitates" was obtained on comparing the results of internal friction measurements with those of metallographic examination.