Experiments conducted with bilayers of elements so chosen as to maximize chemical differences and to minimize physical differences (e.g., atomic weights) demonstrate the importance of chemistry in determining the amount of interaction which results from ion mixing experiments. These results emphasize the dominance of chemical factors which are at least neglected, if not totally ignored, in the modeling of ion mixing effects. Ion bombardment of Pd/Hf bilayers causes the formation of Pd3Hf; the existence of such compounds was not detected in other bilayers of ‘‘similar’’ elements.