The activities of 218Po, 214Po and 211Bi were determined in samples obtained of soil/gas. Sampling work was taken in Jáchymov (Czech Republic) at the outcrop of the Geister-vein, by electrostatic precipitation from filtered soil gas on stainless steel disks. The samples were measured in a field laboratory using a semi-conductor alpha-spectrometer. The activities of 218Po, 214Po and 211Bi were calculated. Samples taken from active dump material (near-by radon source) exhibited a high 211Bi/214Po ratio, while those of the vein outcrop (a relatively deeper source) had a low ratio. A mathematical model was employed to determine the radon age calculated from the actinon/radon input ratio. This varied in a range of 5.6 to -7.7 seconds. Negative age values are probably caused by the preference for actinon, which rapidly comes into equilibrium with the source of this gas. © 2005 Akadémiai Kiadó.