Thin titanium layers (approximately 10 nm) have been grown on top of a clean Si(111) substrate. Heating these layers initiates a solid state reaction, yielding an amorphous monosilicide phase at about 350 °C. The kinetics of the solid state reaction has been followed using three-wavelength ellipsometry (340, 450 and 550 nm). A very coarse two-layer model has been applied in the analyses of the measured data: a top layer of pure titanium is consumed by a second layer of TiSi. The dielectric constants of titanium and TiSi are known and the layer thicknesses d1 and d2 have been fitted to the six ellipsometrical angles measured. These analyses reveal a diffusion-limited growth mechanism exhibiting two growth rates: a rapid initial rate followed by a slower final rate. The diffusion coefficient D of the rapid process and its activation energy Ea could be obtained: D = 2 × 10−15cm2s−1atT 370 °CandEa = 0.62 eV The two growth rates have been attributed to silicon diffusion along the grains and diffusion into the grains.