The Ti-single crystal Si interfaces, before and after annealing in argon, were examined by cross section high resolution transmission electron microscopy (HRTEM) combined for the first time with 2nmΦ probe for energy dispersive spectrometry (EDS). HRTEM shows that there is amorphous alloy formation at the Ti-Si interface. The thickness of the reacted layer is ∼1.7nm for single crystal Si, independent of doping level and impurity species such as As and B, and is ∼2.5nm for back sputter-amorphized Si. After annealing at 430°C for 30min, the thickness of the amorphous alloy increases up to ∼11.5nm. High spatial resolution EDS microanalysis has been obtained. The results show that reliable compositions can be deduced at this level since some of the layers are only about 2nm thick. The amorphous alloy formed at the deposition step was found to be Ti55Si45. After annealing, the composition across the amorphous layer varied from about 70%Si near the substrate to about 30%S1 close to the Ti interface. The substrate interface is atomically flat. Interpretation of the behavior in terms of the metastable Ti-Si phase diagram calculated by Holloway and Bormann will be discussed.