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/dm(3)) 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/dm(3) and pH 8.03 to 8.51. The studied types differ significantly in temperature: 24.3-34.5 degrees C in the first type and 62.1-97.1 degrees C in the second type. The analysis of the distribution of oxygen (O-18) and hydrogen (D) isotopes carried out for the first time confirms different genetic nature of these hydrogeochemical water types. The first type has an atmospheric genesis (O-18 = -7.3...-6.2 parts per thousand and D = -51.4...-39.3 parts per thousand), while the second type restricted to the Kien Giang-Bang intersection zone has a deeper source (O-18 = -1.6...-1.3 parts per thousand and D= -22.2...-21.4 parts per thousand). The tritium (H-3) 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 (H-3 = 4.3-11.1), while that of HCO3-Na water may reach more than 1000 years (H-3 = 0.5 TE).
