Reactions between Pt and SiGe alloy have been studied by comparing several structures: Pt/Ge, Pt/SiGe, and Pt/Si-SiGe superlattices. The Ge, SiGe layers and Si-SiGe superlattices were grown on (100) Si substrates by the ultrahigh vacuum/chemical vapor deposition technique. Pt-Ge reactions start around 200 °C, forming PtzGe. This is followed by the formation of PtGe around 300 °C. The Pt-Ge reactions are thus similar to those of Pt-Si. The reactions between Pt and SiGe, however, involve a preferential Pt-Si reaction. At 200 °C, for example, while Pt2Ge is normally seen from the Pt/Ge system, only PtzSi is detected from both x-ray diffraction and Rutherford backscattering measurements. At higher temperatures, both the PtGe and PtSi phases form. This preferential Pt-Si reaction is observed in both Pt/SiGe and Pt/Si-SiGe superlattice structures.