CharacteristicKx rays from thermal neutron fission of U235were studied as a function of the fission fragment mass. With a NaI (Tl) detector viewing the first 1 cm of fragment paths, x rays in coincidence with fission were recorded together with the fragment energies as detected by silicon surface‐barrier detectors. Results are reported for yields ofKx rays as a function of fragment mass. The most probable charge for various masses is estimated.