Diamond is well suited as an x-ray mask material because of its high x-ray and visible radiation transparency and exceptionally high mechanical stiffness. This paper presents the optical, x-ray and morphological properties of a diamond membranes produced on 100mm diameter silicon wafers. Microwave Plasma Chemical Vapor Deposition was used to deposit diamond films on 1 00 mm diameter silicon then back-etching the substrate to form taut 75mm diameter free standing membranes. Raman spectroscopy and x ray diffraction indicates that the films are diamond. X ray diffraction reveals that the films are fine-grained polycrystalline diamond with a grain size of about 20-30 nm and a [220] growth texture. The films contain microcrystalline graphite and non-carbon diamond as indicated by the broad bands at 1355 and 1560 cm, respectively. The FWHM ofthe main diamond peak is relatively broad,8-1 2 cm , indicating a small crystallite domain and strained microstructure, consistent with the x ray diffraction data. The measured x-ray transparency of a 3.25M thick membrane at the Cu Ka line is 99.5% while the optical transparency at the He-Ne laser line is 45 %, uncorrected for reflection losses from both surfaces, absorption and scatter. The transmission in the IR is at the theoretical limit, 71%. The surface roughness, measured with a sub-micron stylus type profilometer, is about 30 nm rms on the side away from the silicon substrate and 20 nm on the side adjacent to the substrate. The stress in the films is tensile with a value of 0.1 1 GPa. The films contain less than 1% hydrogen as determined by nuclear reaction analysis.