Fuel for the high-temperature gas-cooled reactor is in the form of microspheres surrounded by high-density impermeable coatings to retain fission products. Recent irradiation tests indicated that the high-density carbon layer of Biso coatings may become permeable to krypton, xenon, and CO during irradiation. In-reactor gas release measurements showed the particles were impermeable to fission gases at the beginning of the testy but released significant quantities of krypton and xenon after a period of irradiation. Although postirradiation examination by visual, ceramographic, and radiographic techniques indicated that all particles were intact, gas content measurement showed that particles receiving a significant fast fluence contained only a small fraction of the expected krypton and xenon, while those receiving low fluence retained almost all the fission gas. The results of these experiments indicate that the permeability of the coatings is due to fast-neutron-induced structural changes in the pyrocarbon.