Pt‐Au multilayers deposited on a Si substrate were profiled with 2.5, 5, and 8 keV Ar+ ions in order to gain information on the influence of atomic mixing on secondary‐ion‐mass‐spectrometry depth resolution. Collisional mixing and thermal spike mixing of metallic interfaces have been calculated with no adjustable parameters. The collisional mixing is calculated by Monte Carlo simulation and the thermal spike model based on well‐established solid‐state models is used to describe the late phase of the cascade. Experimentally observed broadening of the Au/Pt and Pt/Au interfaces as a function of primary‐ion energy is predicted by the model. The experimental and calculated decay lengths of the trailing edge in Au are greater than in Pt by a factor of 2–3. This difference in interface broadening in Pt compared to that in Au is due to more efficient electron‐phonon coupling and thus more rapid quenching of thermal spikes in Pt than in Au.