A scanning electron microscope and nuclear microprobe have been used to investigate the interaction between germanium and platinum in a lateral diffusion couple. When an island of germanium on a platinum film was annealed at 500 degrees C three distinct phases were observed to form, two inside and one outside the original island. Micro-Rutherford backscattering spectrometry carried out with the nuclear microprobe suggested that these phases were Pt2Ge, Pt3Ge2 and PtGe (with the latter forming outside the original island). Micro-Rutherford backscattering spectrometry also showed that after reaction the platinum was not uniformly distributed throughout the film in the island region, nor was it uniformly distributed in the reaction zone. Information regarding the thickness of the reacted films, and of the distribution of the various components in the film, are readily accessible via micro-Rutherford backscattering spectrometry, thus demonstrating the power of this technique for lateral diffusion studies.