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The evaporation behavior of polonium as one of the most hazardous radionuclides produced in spallation based neutron sources with liquid lead–bismuth targets has been quantified in this study. The normalized apparent vapor pressure, i.e. the Henry constant of polonium over liquid lead–bismuth eutectic was determined in the temperature range relevant for operation of such targets, i.e. 164–500 °C. For comparison and better fundamental understanding, the Henry constant of polonium over pure liquid bismuth was determined in a temperature range of 300–500 °C. The Henry constants of polonium in this temperature range were found to be orders of magnitude higher than expected from earlier studies at higher temperatures. Possible mechanisms responsible for this unexpected behavior are discussed.