An overview of high-temperature gas-cooled reactor (HTR) fission product chemistry and its influence on source terms in core heatup accidents is given. These accidents are risk-dominating for medium-sized HTRs and are characterized by maximum core temperatures of ∼2500°C (2773 K) and a late-starting, slowly proceeding fission product release from the fuel particles. In these accidents, the number of chemical reactions in the core and primary circuit is limited by the low oxygen potential and preferential release of metal from the fuel. The graphite in the core acts as a very powerful barrier to metallic fission products because of its chemisorption action. Cesium iodide (CsI) formation can reduce this sorptive retention for cesium when there is a high cesium burden on the graphite. This is not necessarily expected for small HTRs, which have much lower maximum accident temperatures (1600° C = 1873 K) and a much lower fractional release of fission products from coated particles. In the primary circuit, less efficient chemisorption of fission products on metals occurs. The fission product chemistry in the HTR reactor building is similar to that for other reactor types. Conservatisms in handling fission product chemistry in HTR safety analyses are identified. This leads to the conclusion that, especially for metallic fission products, a significant potential for reduction of the actual core heatup source terms exists. In modern medium-sized HTRs, these source terms are of the order of <1% of the core inventory for cesium, iodine, and noble gases and <0.1% for strontium. For small HTRs, these source terms remain several orders of magnitude smaller.