High-energy resolved X-ray emission spectroscopy was used to study the influence of oxide thickness on the thermally-induced CoSi2-formation in the Co/SiO2/Si system. The Si L-2,L-3, X-ray emission spectra have shown different fine structure dependent on the thickness of the buried oxide layer, the annealing temperature and the time of annealing. A full analysis of silicidation is undertaken on the basis of comparison of the spectra of the samples under investigation with those of reference samples of c-Si, SiO2 and CoSi2. It is found that silicidation in this system, with formation of CoSi2, starts at annealing temperatures above 900 degrees C. Annealing at 1100 degrees C for 10 s was required for formation of the CoSi2 phase in the Co/SiO2/Si system with a 3-nm thick layer of SiO2. It is shown that increasing annealing time from 10 to 45 s leads to formation of CoSi2 for thicker (about 10 nm) layers of SiO2.