The evaporation behavior of Po 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 Po over liquid LBE 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 Po over pure liquid Bi was determined in a temperature range of 300 - 500°C. The Henry constants of Po 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.