The reaction of thin platinum layers with Si(lll) to form platinum silicides was monitored in situ by core level electron spectroscopy and spectroscopic ellipsometry, The chemical composition and stoichiometry of the reaction products were identified at crucial stages of the reaction via photoemission while the kinetics of the reaction could be followed via ellipsometry. Utilizing the optical constants of the reactants Pt and Si and the main reaction products PtSi and a Pt-rich Pt2Si (x similar to 2) an initial room temperature reaction of Pt and Si extending some nanometres into the Pt overlayer has been detected and the evolution of the ellipsometric angles during the reaction of Pt and Si at elevated temperatures could be modeled quantitatively.