In this study we investigate the formation mechanisms and morphology of TiSi2 formed by deposition of Ti on atomically clean silicon substrates. Ti films of 50–400 Å thickness were deposited in ultra-high vacuum on Si (111) wafers and annealed to temperatures between 500–900°C. Films were monitored in situ with AES and LEED, and post preparation characterization was accomplished with SEM, TEM and Raman scattering. The results show that for films of thickness ≤, 100 Å the C49 TiSi2 phase is stable over the entire 600-800°C temperature range. However, for films of 200-400 Å thickness, the C49 to C54 phase transition occurs at temperatures varying from 700 to 800°C dependent upon film thickness. The high temperature annealing results in flat interface structures, and island formation is observed for all films with the C54 structure. The interface morphology and the mechanisms of TiSi2 island and phase formation are discussed in terms of surface and bulk free energies considerations based on nucleation theory.