Thin copper films of about 15 nm thickness were deposited on Si(1 1 1) substrates at room temperature and subsequently annealed at higher temperatures. The ellipsometric analysis was performed in situ under UHV conditions in the wavelength range 400-800 nm. The as-deposited films can be described by a two-layer model taking into account the excitation of surface plasmon polaritons (SPP) at the rough copper-silicon interface. During annealing, two steep rises in the ellipsometrical angles Delta and psi are observed at 125 and 300 degreesC which are attributed to the coagulation of copper and the silicide formation, respectively.