Measurements of the internal friction in pure Al and two aluminium alloys with 2 and 2.95 wt % Cu were made using a torsional pendulum. With solution‐treated and quenched alloys a weak internal friction peak (peak I) was obtained at the same temperature range at which grain boundary relaxation occurred in pure Al. The peaks disappeared after a few minutes of aging due to the blocking of grain boundary relaxation on precipitation. The activation energyHcorresponding to this peak is 32±1 kcal/mole for both the 2 and 2.95% alloys. A grain boundary peak also appeared in furance‐cooled 2% alloy withH= 31±1 kcal/mole, but instead of a peak an inflexion point appeared in 2.95% alloy. For pure Al,H= 32.4 kcal/mole was obtained. Due to re‐solution of the precipitates at higher temperatures another strong internal friction peak (peak II) appeared a few degrees below the solubility temperatures for both the alloys. The position of the peak is insensitive to the frequency of measurements but the peak height decreases on increasing the frequency. From the rigidity modulus measurements the solubility temperatures for the 2 and 2.95 wt % Cu in Al are obtained and are found to be in good agreement with previous results. An internal friction peak due to Zener relaxation, however, has not been observed in these alloys.