Commercially pure molybdenum specimens were irradiated in the Materials Testing Reactor for an estimated exposure of 1.9 to 5.9×1020 thermal nvt. Prior to irradiation, the material was ductile in the tension test; whereas after irradiation, it was brittle. The results of tension tests conducted at various temperatures revealed that the transition temperature for this material had been increased from −30° to +70°C as a result of the radiation exposure. From metallographic studies, it is concluded that the embrittlement is due to submicroscopic changes which raise the flow stress-temperature curve of the material.