A method and a device for decontaminating optical surfaces, in particular for decontaminating the surfaces of beam-guiding optics employing UV-radiation in a cleansing atmosphere. The wavelength of the UV-radiation employed falls within a range where oxygen strongly absorbs and the cleansing atmosphere has an oxygen concentration less than that of air. The method and device have application to, e.g., cleaning the surfaces of the beam-guiding optics of microlithographic projection-exposure systems.