Measurements have been made of the change in residual electrical resistivity in thin‐film specimens of Cu and Ag induced by light‐ion irradiations (H,D,3He,4He) below 10 K in the energy range 15–40 keV. The number of Frenkel pairs created per incident ion was deduced and compared with theoretical predictions. The efficiency factor (ratio of experimental to theoretical value) was found to decrease from ∼1 to 0.7 (0.9 to 0.65) for Cu (Ag) as the recoil spectrum was shifted to higher energies by increasing the projectile mass and/or energy.