Low‐temperature ion beam mixing rates for Ni‐Ti, Zr‐Ni, and Pd‐Ni bilayers significantly exceeded binary collision estimates, and appeared quite sensitive to thermodynamic driving forces. In the absence of a temperature dependence such a behavior is commonly ascribed to interdiffusion within thermal spikes. However, the Ni‐Ti mixing rate was seen to vary linearly with nuclear damage energy for irradiation with 600 keV Xe, Kr, or Ar, 300 keV Ne or N, or 200 keV N ions, or 1 MeV Au ions (literature value). This excludes overlapping thermal spikes. An expression was derived for mixing due to nonoverlapping thermal spikes, but this could also not explain our results.