Single-wavelength ellipsometry is employed to monitor in situ the reaction of 23 nm platinum layers with Si(100) to form platinum silicides during heating at constant rates up to 100 K/min. A previous study showed that by the use of 'Kissinger' plots the activation energy of the intermediate silicide phase Pt2Si and that of the final PtSi can be determined with an accuracy of 50 meV. It is the purpose of the present paper to extend the study one step further and obtain also the pre-exponential growth factor by directly modeling the evolution of the ellipsometric data as they were obtained during the temperature ramp. To do so, the chemical composition of the reaction products and the thickness of the formed layers were identified at crucial stages of the reaction via RES. Additionally, the optical constants of the constituent phases Pt, Pt2Si and PtSi were calculated from spectroscopic ellipsometry measurements in the range from 1.5 to 4.5 eV. With this input information the evolution of the ellipsometric angles during the reaction of Pt and Si were modeled quantitatively. The analysis yields good fits to the data for different ramp rates with activation energies of 1.50 and 1.70 eV and reaction coefficients of 37 and 27 cm(2)/s for the Pt/Si to Pt2Si and Pt2Si to PtSi formation, respectively. A significant improvement of the fit is obtained when the activation energies are allowed to be distributed about their mean value with a width of 47 meV. This distribution takes inhomogeneities in the reaction process into account and leads to a broadening of the reaction fronts.