Irradiation tests to evaluate coated-particle fuels under high temperature, high burnup, and high fast-neutron dose conditions were designed so that the irradiation parameters could be measured and controlled closely. The coated particles used consisted of two-layer (BISO) and threelayer (Triplex) pyrolytic carbon coatings on fuel particles of the carbide or oxide of thorium and/or uranium. In the irradiation experiments, the coated particles were tested at temperatures up to 1900°C, fuel burnups up to 20% fissions per initial heavy metal atom (FIMA), burnup rates up to 20% FIMA per month, and fast-neutron doses up to 2.7 x 1021 n/cm2 (E > 0.18 MeV). Correlations between the irradiation temperatures and other important radiation and materials parameters are presented showing the effects of these parameters on the stability of the BISO and Triplex coated fuel particles. These studies show that the temperature of irradiation is one of the most important parameters influencing coated-particle fuel stability under irradiation. This is believed to be due to the dependence of the fission gas pressure on irradiation temperature and the deleterious effect that an increase in this pressure has on the coating. Thicker coatings are required for high-temperature operation. The studies also demonstrated that coated particles with two- and three-layer pyrolytic-carbon coatings and adequate fuel contents have good stability well beyond the temperatures anticipated in an HTGR.