Solid State interactions have been found to take place at room temperature in a large number of bimetallic thin film couples. These couples consisted of a layer (-3000 Å) of Pb or Sn and a layer (-5000 Å) of a noble or transition metal, and were prepared by consecutive evaporations onto fused quartz substrates maintained at room temperature. Intermetallic compounds that formed as a consequence of annealing these couples at room temperature have been identified by X-ray diffraction and follow a rule that only one compound formed predominantly over others and the compound is the one whose composition is richest in Pb or in Sn. The formation rates are so fast that we cannot explain them without assuming an extremely fast diffusion, most likely by an interstitial mechanism, of noble and transition metals in Pb and Sn. © 1974 IOP Publishing Ltd.