Thin (about 10 nm) titanium layers have been grown on clean Si(111) surfaces inside a UHV system and subsequently heated. The Ti---Si solid state reaction initiated is followed by three-wavelength ellipsometry. Successively we observed two metastable phases at about 350 and 450 °C (states I and II) prior to a final transition at about 700 °C (state III). The as-deposited layers, the intermediate states I and II, and the final state III have been characterized by means of spectroscopic ellipsometry (E = 2.0−4.5 eV). Analysis of state I reveals an inhomogeneous, probably silicon-enriched monosilicide. State II is again homogeneous and it has an almost stoichiometric composition of TiSi2. Most probably it is the C49 disilicide phase. Optical analysis of the final state III reveals a thick layer of C54 TiSi2 islands containing 20%–25% c-Si precipitates. Reference samples identical to states I, II and III were made and investigated by Rutherford backscattering spectroscopy. Both thickness and composition results obtained from these measurements fully support the results of the ellipsometrical study.