An ultraviolet absorption feature has been seen in solid deuterium-tritium and hydrogen-tritium at a sensor temperature of 5 K. The peak occurs at 3.6 eV and is about 1.5 eV wide. It disappears when the temperature is raised to about 10 K but reappears upon cooling and is, therefore, radiation induced. At 5 K, the absorption line forms on a time scale of minutes and appears to represent part-per-million levels of electron-mass defects. The suggested model is that of a trapped electron, where the absorption line is the ground state-to-the-conduction band transition. A marked isotope effect is seen between D-T and H-T.