The elastic constants of a Cu—4 atom percent Si crystal have been determined by the pulsed ultrasonic method to beC11=1.62,C12=1.20, andC44=0.755, all in units of 1012dyne cm−2. The changes in the fundamental shear constantsC44and ½(C11–C12) owing to alloying have been expressed in terms of a change in electrostatic stiffness and a change in ionic stiffness, both of which are shown to be reasonable. The experimental ionic effect is interpretable in terms of the solute‐solvent ion core interaction and the local distortions around a solute atom. The previously known results for Cu—28 Zn also conform to this picture.