Abstract—The world-famous Tskaltubo spa-resort is located in western Georgia 250 km from the city of Tbilisi, and 90 km from the Black Sea.The healing component of this spa-resort, is warm (30–34 ℃) natural springs containing the noble gas radon. The current view of the waters is that they are of hydrothermal origin, and their radon enrichment occurs in the quaternary sediments (1–8 m thick) of the Tskaltubo depression. The content of uranium and thorium (parent elements to radon) in Quaternary sediments of the Tskaltubo depression was studied under both field and laboratory conditions. The results indicate that the Quaternary sediments of Tskaltubo have relatively low radiation (less than 0.17 μSv/h), and typical crustal levels of U and Th (less than 1.4 ppm and 5.3 ppm, respectively. Therefore, the Quaternary sediments cannot be the source of elevated radon. We believe that Bajocian porphyritic igneous rocks located at a depth of 700–800 meters beneath the Tskaltubo depression, do have radiation levels from (120 mkr/h to 220 mkr/h) that are close to that expected from the radon concentration in the spa waters. From this zone, the upward flow of radon mixes with the groundwater of the Lower Cretaceous limestones, which flows into the Tskaltubo depression through a diabase dike barrier. The heating of the springs to a temperature of +30–34 ℃ is still unclear: the waters become warmer as they get closer to the surface unlike a more typical deep geothermal source. Although thermal energy released during the radioactive decay of radon (²²²Rn) to the stable isotope of lead (²⁰⁶Pb) would increase water temperature, radiation levels do not account for the full thermal range observed.