Inhomogeneous media such as ceramic-metal composites are used in a wide range of applications as functional optical coatings e.g, in selective solar absorber coatings. Their optical properties can be calculated by the use of effective medium theories. These theories need the optical constants of the components as input parameters. Especially for the metallic component, these parameters depend strongly on the deposition conditions and on the particle sizes. The limitation of the mean free path of the conduction electrons can be explained by the well-known Drude theory. It was found that additional size effects occur due to a size-dependent modified band structure of the metal particles. The size dependence of the band structure and the resulting effects on the optical constants are discussed for th.e model system of sputtered small gold particles embedded in an amorphous carbon matrix produced simultaneously by hydrogenated, plasma- activated, chemical vapour deposition. Changes in the band structure were analysed by ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS). The optical constants of the cermets were determined from reflectance and transmittance measurements as well as from spectral ellipsometry. Changes in the optical constants of the gold particles were found from comparison of these measurements with effective medium theories. The size of the metal particles and the surface structure of the cermets were determined from TEM images and atomic force microscopy. The size-dependent changes of the optical constants of the metal particles were correlated with the UPS spectra.