The phase development sequences for Pt layers on Si, Ge, and Si-Ge at 327°C were studied by X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy. The reactions on pure Si and pure Ge followed the sequences reported previously. The reactions on the alloy layers differed slightly from the simple binary reactions. A ternary Pt(Si,Ge) reacted layer was formed on the alloy samples as the final phase on the Si-Ge alloy layer. This is discussed in terms of enthalpies of formation and the relative diffusion rates of Pt, Si, and Ge.