The formation of NiSi films on Si was studied using Rutherford backscattering spectrometry, atomic force microscopy, and ellipsometry. NiSi is an attractive candidate for use as a gate contact material due to its low metal-like resistivity and large processing window (350-750 degreesC). Three phases, Ni2Si, NiSi, and NiSi2, were identified in this temperature range, and their optical databases in the 2-4 eV photon range were established, and used to model real-time ellipsometry data. It is shown that real-time ellipsometry can be used to monitor and follow the formation of the various Ni-Si phases. We have also observed the onset of agglomeration of the silicide for longer time anneals at temperatures of 500-700 degreesC, which is much lower than 1000 degreesC where agglomeration has been reported to occur.