The reliable use of TiSi2 in integrated circuit metallizations will depend in part on an intimate knowledge of the effects of processing variables — such as annealing time, temperature, and ambient — on the phase formation sequence. For this reason, transmission electron microscopy has been used to investigate the formation of TiSi2 thin films on silicon substrates [1]. For films formed either by reacting titanium with a silicon substrate or by sintering a codeposited (Ti + 2 Si) mixture, we find that a high resistivity (∼60 μΩcm), metastable phase — TiSi2(C49 or ZrSi2 structure) — forms prior to the desired low resistivity (∼15 μΩcm), equilibrium phase — TiSi2(C54 structure). In titanium-silicon diffusion couples, a thin layer of TiSi is also present on top of the metastable TiSi2- For processing temperatures above 550°C, the available data suggest that the metastable TiSi2 forms first and acts as a template for subsequent nucleation of the TiSi phase. In codeposited (Ti + 2 Si) films, TiSi2(C49 structure) is the only intermediate phase. The temperature at which the C49 structure transforms to the C54 structure increases significantly as the film impurity content increases. Differences in earlier reports of TiSi2 formation [2–5] appear to be resolved if x-ray diffraction peaks attributed to Ti5Si3 and TiSi were actually from the metastable TiSi2