The phenomenon of Strong Metal-Support Interactions (SMSI) has been explained previously in terms of a chemical bonding between dispersed metal atoms and cations of certain supports. In this study, planar specimens of Pt Al2O3, Rh Al2O3, Pt TiO2, and Rh TiO2 were heated in hydrogen or oxygen and the resulting changes were observed by the technique of Nuclear Backscattering Spectrometry (together with electron microscopy). Significant and complex diffusion effects were found to occur, being dependent on the particular metal/support combination plus the environment and temperature to which it is exposed. This goes beyond the original mechanistic concept and the results suggest a much wider range of SMSI processes. © 1983.