Abstract: There is a need in the field of x-ray optics for an oxygen-free polymeric filter. This is because the mass absorption coefficient of oxygen is greater than that of carbon at every energy in the soft x-ray region except for a narrow band between the oxygen k-edge at 543.1 eV and the carbon k-edge at 284.2 eV. This band of frequencies is known as the "water window." The present invention is a method of filtering x-rays with the use of a freestanding, thin film of a polymer, metal-polymer composite, mesh-supported polymer film, or mesh-supported metal-polymer composite where the polymeric component of the filter is a PBI or a PPQ. In accordance with the present invention, the soft x-ray transmission of a 2500 angstrom polybenzimidazole film was measured as 3 to 5% more transmissive than either the polyimide DuPont 2610D or the polycarbonate GE Lexan.