Abstract: ~(222)Rn dissolved in the water is determined using nuclear track detector (NTD) technique in atwofold manner: the α--sensitive NTD CR--39 is used to measure the α--activity of ~(222)Rn. This α detectoris calibrated by simultaneous γ measurement of the daughter nuclides ~(214)Pb and ~(214)Bi, which is in ra-dioactive equilibrium with ~(222)Rn. This technique was applied to determine quantitatively the ~(222)Rn con-centration as a function of depth of the KTB drill--borehole down to 2800 m.