[en] Isotope composition (deuterium, ¹⁸O, ⁸⁷Sr and ³He) of thermal and carbon-dioxide waters widespread in Mongolia was studied. According to deuterium and heavy oxygen, the waters under study are close to the meteoric standard of the territory. The isotope composition of both meteoric and mineral underground waters is distinctly seen from north to south, from mountain taiga to desert plain. The Sr isotope composition of mineral waters have the maximum ⁸⁷Sr/⁸⁶Sr ratios in the north and in the central part of the territory (South Hangi), the carbon-dioxide waters have low values of this parameter. Elevated values of ³He/⁴He are typical for the underground (thermal and fresh) waters of Hubsugul region and CO₂-waters of East Mongolia, whereas the nitrogen therms of Hangai are ³He-depleted. An inverse correlation exists between ³He/⁴He and total content of He in the mineral waters. Based on isotope determinations and other geological data, genesis of Mongolian thermal and carbon-dioxide waters is discussed. 4 refs., 6 figs., 3 tabs