The fission gas release from three (U,Pu)O2 fuel specimens was measured during irradiation in a sweep-gas experiment. Two of the specimens were made from sintered powder of the same composition, but one specimen was pellets and the other microspheres. The third specimen was sol-gel microspheres. The specimens all showed a decrease in fission gas release during the initial portion of the irradiation. This, we believed, was caused by irradiation sintering of small internal passages. The pellet specimen suffered an almost explosive breakaway gas release when the specimen temperature was suddenly raised from 1100 to 1450°C. The sintered microspheres were irradiated at temperatures just at the onset of breakaway gas release and a relation between burnup and temperature for breakaway gas release was established. About 10% of the sol-gel microspheres contained large internal voids that were not detectable by pre-irradiation optical inspection. The gas release from those with voids was large enough to obscure the gas release from the remainder of the sol-gel microspheres. In general, the fission gas release from all three specimens was about an order of magnitude higher than that expected for comparable specimens of UO2.