Abstract: Study of the δ18O and δD data of geothermal waters in the Guanzhong basin indicates that: significant 18O and deuterium exchanges took place in geopressured geothermal waters at depths beneath Xi′an and Xianyang cities in the central part of the basin, suggesting that the geothermal reservoir fluids have undergone strong water-rock reaction; whereas the 18O exchange in non-geopressured geothermal waters at peripheries of the basin and in its central part is not evident. Based on the degree of 18O exchange, (expressed by the deuterium excess parameter d) and hydrochemical data in the area, two geothermal water reservoirs may be distinguished in the Guanzhong basin; they are circulating (current) geothermal reservoirs and isolated (fossil) geothermal reservoir. With increasing buried depth, residence time, total dissolved solids and temperatures of geothermal waters and more isolation of the geological environment of geothermal waters, the degree of 18O exchange becomes higher. Geothermal waters of Xi′an and Xianyang belong to two different geothermal water systems and are recharged by difference sources.