Measurements of the rate of uranium atom emission from a fissioning surface were made on uranium and uranium oxide sources. A technique was employed in which the sources could be maintained in vacuuo at all times during the experiments. It was found that at the start of an experiment the rate of emission was high, but declined with increasing reactor exposure to a low constant value of ≈6 uranium atoms ejected per fission fragment penetrating the surface for the metal and ≈38 for the oxide. Exposure of a uranium metal source to moist helium, after a constant emission rate was achieved, led to a suddenly increased rate that tended to return to that obtaining before treatment.