The formation of Pt silicides in the presence of Al is studied using Al/Pt/Si structures with different layer thicknesses of Al. Both the Pt-Si and Pt-Al reactions take place simultaneously, starting around 200 °C, with the formation of PtSi and PtAl2 completed at 400 °C. With an equal layer thickness for all three layers, the two alloys formed stay in contact with each other in a stable form up to about 500 °C. With a layer thickness of Al twice or more that of Pt and Si, the PtSi layer is completely converted to PtAl2 at 450 °C. The released Si from such a reaction is mostly located under the PtAl2 formed. The relation between these results and the unstable PtSi/Al contact in devices is discussed.