III situ ellipsometry is employed to study the kinetics of Pt silicide formation by thermal annealing with constant heating rates up to 30 K/min of a 10 nm platinum layer on silicon (100). From spectroscopic ellipsometry data of the initial Pt/Si, the intermediate Pt2Si, and the final PtSi phase an optimum photon energy of 3.5 eV was chosen for the kinetic study. Characteristic changes of the ellipsometric angles are observed as a function of temperature when the reaction fronts reach a depth below the surface that is related to the absorption length of the 3.5 eV photons. From an analysis of these transition temperatures based on the Kissinger formalism the activation energies for the formation of Pt2Si and PtSi are obtained as 1.55 +/- 0.05 and 1.72 +/- 0.05 eV, respectively, where the error bars follow from the precision of our measurements. A comparative study for a 100 nm Pt layer on Si yields the same activation energies albeit with a higher uncertainty, indicating that this method is particularly suited for ultrathin silicides.