The reaction of platinum films about 450 Å thick which had been vacuum deposited onto Ge〈100〉 substrates was investigated by backscattering spectrometry and X-ray diffraction. It was found that the procedure for cleaning the germanium is critical. If the cleaning is inadequate, no reaction is observed on thermal annealing in vacuum up to 300-350°C, and a laterally non-uniform reaction starts suddenly above that temperature. A cleaning process is described that reproducibly leads to laterally uniform compound formation above 250°C. The compounds Ge2Pt3, GePt, Ge3Pt2 and Ge2Pt are observed by backscattering spectrometry in sequential order for 30 min annealings up to 450°C. The GePt phase formed at 300°C was identified by X-ray diffraction. These results are relevant to the formation of contacts to high purity germanium radiation detectors.