Abstract—New isotope hydrogeochemical data on two types of thermal water from the Bang spring (Kuang Binh province, central Vietnam) are reported in the work. The first type includes HCO3–Cl–Na–Mg water with extremely low mineralization (44–87 mg/dm3) and pH variations from weakly acidic to weakly alkaline values (5.71–7.84). The second type includes HCO3–Na water with mineralization up to 256–659 mg/dm3 and pH 8.03 to 8.51. The studied types differ significantly in temperature: 24.3–34.5°C in the first type and 62.1–97.1°C in the second type. The analysis of the distribution of oxygen (δ18O) and hydrogen (δD) isotopes carried out for the first time confirms different genetic nature of these hydrogeochemical water types. The first type has an atmospheric genesis (δ18O = –7.3…–6.2‰ and δD = –51.4…–39.3‰), while the second type restricted to the Kien Giang–Bang intersection zone has a deeper source (δ18O = –1.6…–1.3‰ and δD = –22.2…–21.4‰). The tritium (3H) data also point to different circulation times of these waters. The formation time of the HCO3–Cl–Na–Mg waters does not exceed 50 years (3H = 4.3–11.1 ТЕ), while that of HCO3–Na water may reach more than 1000 years (3H = 0.5 ТЕ).