An experimental arrangement is described which allows the deposition of thin gold films on Si(lll) single-crystals and the in situ optical characterization at various temperatures, thicknesses, and angles of incidence of the light. The optical analysis is performed with the help of a highly sensitive spectroscopic ellipsometer operating in the wavelength range 400-900 nm. Additional structure information is obtained from REM, LEED/Auger, X-ray and resistivity measurements. The gold films exhibit a homogeneous single-crystal structure after deposition at room temperature. With increasing annealing temperature the films crack and form an island-shaped eutectic Au/Si melt, cooling leads to a decomposition of the binary mixture and formation of gold islands embedded in the silicon surface. The eutectic temperature is found to be smaller than in bulk samples with a marked hysteresis in the Delta or psi vs. T curves. The wavelength dependence of psi shows, in addition to the well-known interband transition of gold, a resonance at about 570 nm. It can be traced back to a Mie plasmon excitation in the solid gold islands which disappears in the liquid state.