Abstract: The invention relates to a method for preventing contamination of the surfaces of reflective optical elements for the soft X-ray and EUV wavelength range during their irradiation at operating wavelength in an evacuated closed system having a residual gas atmosphere, said elements comprising a cover layer consisting of at least one transition metal. According to said method a residual gas atmosphere is adjusted. The aim of the invention is to prevent a degradation of the surfaces by deposition of carbon and by surface oxidation. For this purpose, both a reducing gas or gas mixture and a gas or gas mixture containing oxygen atoms are introduced.