SiO x films (1<x<2), 0.5μm thick, have been elaborated by electron-gun evaporation. A thermal annealing of these films induced a phase separation leading to the formation of Si nanocrystals embedded in a SiO 2 matrix. These films have been studied by infrared spectroscopic ellipsometry and by X-ray photoelectron spectroscopy (XPS). The effective dielectric function of the thin films has been extracted in the 600–5000cm −1 range which allowed us to deduce the dielectric function of the matrix surrounding the Si-nc. A study of the Transverse Optical (TO) vibration mode has revealed the presence of SiO x into the matrix. Before XPS measurements, the films have been etched in fluorhydric acid to remove the superficial SiO 2 layer formed during air exposure. The Si 2p core-level emission has been recorded. The decomposition of the Si 2p peak into contributions of the usual five tetrahedrons Si-(Si 4− n O n ) (n=0–4) has also revealed the presence of a SiO x phase. Consistency between infra-red and XPS results is discussed.