Theoretical and experimental aspects of cesium release from Biso-coated high-temperature gas-cooled reactor fuel particles are reviewed. Three types of transport are considered: diffusion through the pyrolytic carbon (PyC) coatings under conditions of a constant source potential, transport due to a continuously depleting source, and diffusion from a depleting source following constant source potential transport. Analysis of all of the release data obtained in this laboratory yields the following relations for the diffusion coefficient DCs characteristic of cesium transport through PyC coatings:For coatings deposited from propylene or propane at ∼1250°C (1523 K):and far coatings deposited from methane at ∼2000°C (2273 K):where T is the absolute temperature.