Single-wavelength ellipsometry is employed to monitor in situ the reaction of titanium layers of different thickness (10-80 nm) with Si(100) to form titanium silicides during heating at constant rates up to 100 K / min. Previous studies performed on the platinum / silicon system showed that by the use of 'Kissinger' plots the activation energy for the formation of the intermediate silicide phase Pt Si and that of the final PtSi can be determined with an accuracy of 50 meV. Additionally, by 2 directly modelling the evolution of the ellipsometric data as they were obtained during the temperature ramp the pre-exponential growth factor of the two reactions was determined as well. In the present study the reaction of Ti with Si to the intermediate C49-TiSi phase was followed and the activation energy of this reaction was determined to be 2.30 eV. The 2 isomorphous C49-/ C54-TiSi phase transition can also be traced by ellipsometry. However, the change in the ellipsometric 2 angles is less pronounced which might be ascribed to a TiO overlayer. Complementary to the ellipsometric analysis of the x silicidation, the chemical composition of the reaction products and the thicknesses of the formed layers were identified at crucial stages of the reaction by X-ray diffraction, Raman spectroscopy, and Rutherford backscattering spectroscopy.