In the course of the German high-temperature gas-cooled reactor Prototype Nuclear Process Heat Safety Program, the behavior of unirradiated fuel particles as well as of irradiated fuel elements at high temperatures was investigated. Unirradiated fuel particles with different designs have been heated to temperatures of 2500°C. Different particle types showed a different high-temperature behavior. While the Biso Thorium High-Temperature Reactor (THTR) type was the most resistant one, Triso particles failed at lower temperatures because of the SiC decomposition. Whole fuel spheres with Biso particles, irradiated in a pebble-bed reactor, were also heated up to 2500°C THTR fuel elements with a burnup of 12 to 16% FIMA (120 000 to 160 000 MWd/t) showed excellent behavior up to 2400°C. At 2500°C, the particles failed in significant numbers after some hours. While rare gas nuclides and iodine were retained in the coated particles as long as the coatings remained intact, the release of some solid fission products, especially cesium, was high above 2000°C.