Properties affecting the irradiation performance of outer pyrolytic carbon (PyC) layers on Triso- and Biso-coated fuel particles were studied. Irradiation temperatures were 1000 to 1500°C (1273 to 1773 K). Fast-neutron fluences reached 12.4 × 1025 n/m2 (E > 29 fJ)HTGR, which is 55% beyond the large high-temperature gas-cooled reactor peak design exposure of 8.0 × 1025 n/m2. Coatings with densities between 1.85 and 1.95 Mg/m3 and mean optical anisotropy values of ≤1.03 (BAF0 units) exhibited the best irradiation performance on Triso particles. For Biso particles, it is necessary to deposit the outer layer at coating rates between 3 and 8 µm/min and with densities ≥1.84 Mg/m3 to produce coatings impermeable to fission gases after irradiation. Data from fuel rod tests show that it is important to limit the degree of surface-connected porosity of the outer PyC layer and the amount of binder phase in the matrix to prevent coating failures resulting from coating-matrix interactions.