Four irradiated UO2 samples were ground to break them along grain boundaries, then dissolved in a series of successive leaches with 3N HNO3. The successive acid extractions were then analyzed for Cs138, Ce144, Zr95, Sr90, and Eu155, as well as total uranium. Very considerable variation in the specific activities of the fission fragments was found between one acid extraction and another of the same sample. The fission products were concentrated in the first and last portions of the material to dissolve. In one sample, which underwent irradiation for a very short time, the increase in concentration in the last portion to dissolve was not observed. It is speculated, on this and other evidence, that fission fragments do not remain in solid solution in uranium dioxide, but instead migrate to grain boundaries and other lattice defect sites.