The important parameters controlling the anisotropy of the Zener relaxation in quenched and reverted Al-4% Cu (1.8 at. % Cu) have been determined from internal friction measurements in torsional and transverse vibration on 〈100〉 oriented single crystal specimens. A new flexure pendulum apparatus, devised for the measurements in transverse vibration, is described briefly. The results show that the anisotropy is not only much larger than is predicted by the theory of Le Claire and Lomer, but that the maximum tensile relaxation strength occurs in the 〈100〉 orientation, for which this theory predicts a minimum value. The pair-reorientation theory of Zener is also applied to the results from this dilute alloy, and is shown to fail in the same way as Le Claire and Lomer's theory. So far, only the suggestion of Seraphim and Nowick, that the relaxation involves primarily the réorientation of second neighbor pairs, appears to be capable of explaining the observed results. © 1961.