The paper gives the results of an isotope investigation carried out on the waters of the crater-lake, Lake Saint Ana (Sf. Ana), of the Ciomadul Mountain, which is located in the Harghita Mountains of the East Carpathians in Romania. The main purpose of this study was to ivestigate the isotope signature of the lake water. The collected water samples were examined for stable carbon, oxygen and hydrogen isotopes, and for noble gas concentrations, and, in addition, for radiocarbon and tritium. Sediment samples were collected for tritium measurement. The measured stable oxygen and hydrogen shows that the lake was subject to mixing before the sampling and there is ~50 % evaporation loss. The stable carbon analysis shows that there is an abundant volume of volcanic degassed CO₂ surplus which can be derived from the mantle. The ¹⁴C activity of the lake’s dissolved inorganic carbon is significantly less than the ¹⁴C content of recent atmospheric carbon. This reservoir effect is supposed to be caused by the volcanic degassed CO₂ with no ¹⁴C content. Depletion of radiocarbon in the order of 85–92.5 pMC could indicate a reservoir age of 950–1650 years. The tritium values of the lake water confirm the mixing theory of the lake and the tritium content of the sediment pore water shows that the lake discharges into the Ciomadul Mountain. The helium isotope ratios indicate a minor helium flux of mantle origin.