Helium permeability of pyrocarbon (PyC) coatings of Biso fuel particles is evaluated by measuring helium content of fuel particles after heat treatment in a helium atmosphere. Fuel particles were annealed in a vacuum at 1800°C (2073 K) to remove any process helium before being annealed in a helium atmosphere. Helium release was monitored as the particles were individually broken in a vacuum chamber connected to a mass spectrograph sensitive to 10−14 mole of helium. Typical particles annealed in helium at atmospheric pressure contained 10−10 mole of gas. The inert gas content of the buffer layer is completely released when the PyC layer is cracked. Helium permeability was strongly temperature dependent and varied considerably with the properties of the PyC coating resulting from deposition conditions.