Fires can mobilize radionuclides from contaminated biomass through suspension of gases and particles in the atmosphere or solubilization and enrichment of the ash. Field and laboratory burns were conducted to determine the fate of I, Cs and C1 in biomass fires. Straw, wood, peat, dulse (seaweed) and radish plants were combusted with temperatures varying from 160 to 1000°C, representing the normal range of field fire temperatures. Loss to the atmosphere increased with fire temperature and during a typical field fire, 80-90% of the I and C1, and 40-70% of the Cs was lost to the atmosphere. The remainder was left behind in the ash and was soluble. Typically, the ash was enriched in I by a factor of two to three, with higher enrichments of Cs and lower enrichments of C1, when compared to the initial fuel concentration during field burns. Most of the I was lost to the atmosphere as a gas. If the elements were radioactive isotopes, such as 129I, 137CS and 36C1, fires could cause an increased radiological dose to people through inhalation, exposure to ash, or ingestion of plants because of increased uptake of ash leachate.