In this paper we report on the radiation stability of silicon carbide and diamond membranes as potential x‐ray lithography mask carriers. Silicon carbide membranes have been produced by epitaxial chemical vapor deposition (CVD). Diamond membranes have been produced in tension with diameters up to 2.5 cm using a microwave plasma CVD process. The stability of these membranes upon exposure to x rays has been studied. The materials were characterized before and after irradiation for their stress, Young’s modulus, infrared absorption from 2.5 to 25 μm, and visible transmission from 300 to 800 nm. The x‐ray exposures were performed using the Center for X‐ray Lithography beamlines at the Aladdin synchrotron at the University of Wisconsin. These beamlines provide a broadband x‐ray spectrum from 900 to 5000 eV. Incident doses of 6700 and 2400 J/cm2 were delivered to the silicon carbide and diamond samples, respectively. Both materials were found to be stable for these exposure levels within the accuracy of the experimental methods.