The fission-gas release from pyrolytic-carbon-coated fuel particles was measured during irradiation, and the damage to the coating material and to the fuel was studied by postirradiation metallography. These particles were either uranium oxide, uranium carbide, or thorium-uranium carbide with a porous carbon primary coating. Particles coated with dense pyrolytic carbon and those coated with a combination of pyrolytic carbon and silicon carbide layers performed well during irradiation in the 1250 to 1400°C range, but both suffered severe internal reactions in the 1650 to 1700°C temperature range, even at low burnup. With one exception, all the experiments were conducted at a much higher burnup rate than would be encountered under normal power reactor conditions.