Abstract: Gaseous carbon dioxide (CO2 ) in natural gas and dissolved inorganic carbon (DIC) in aqueous system are in chemical and isotopic equilibrium. A gas-water exchange experiment with inorganic gaseous CO2 in a closed glass bottle was performed to elucidate the meticulous dynamic change in the chemical composition when gaseous CO2 dis- solves in the water. The results indicate that the CO2 gas achieved compositional and carbon isotopic equilibrium through the gas-water interface within 13 h at 101 325 Pa and 30 ℃ . The initial concentration and δ13 CCO2 values of gaseous CO2 were 2. 0% and -2. 5‰,respectively,and the equilibrium values were 1. 1% and -14. 6‰,respective- ly,with a strong isotopic fractionation (i. e. ,12. 1‰). During the gas-water exchange and dissolution of CO2 ,the CO2 concentration and δ13 CCO2 values could be expressed as an exponential function of time. The δ13 CDIC value,which was approximately -8. 2‰,indicated that a larger proportion of DIC came from organic origin. Gaseous inorganic CO2 was mixed by organic CO2 in water through the gas-water interface,which ultimately presented the characteristic of organic gas. Therefore,carbon isotopic geochemistry of groundwater must be investigated prior to determining the origin and source of gaseous CO2 in natural gas.