Geothermal waters from the Tertiary aquifers located at 1000-3000 m beneath Xi'an city, Shaanxi Province, China, show unique isotopic composition as compared to local groundwaters from shallower Quaternary aquifers. Positive oxygen shifts of as much as 8 parts per thousand VSMOW are observed, while the corresponding 82 H values remain essentially constant at about -80 parts per thousand VSMOW, which is significantly different from those of waters in the Quaternary aquifers with a mean 62 H value of -60 parts per thousand VSMOW. The strong O-18 shift is a result of isotope exchange between geothermal water and carbonate minerals such as calcite over a residence time of several thousand years up to 30,000 years, based on C-14 dating. A comparison of the isotopic composition of geothermal waters with neighbouring groundwater units on both sides of the Guanzhong Basin indicates that the geothermal reservoirs are recharged by rain that falls on the northern slope of the Qinling Mountains, south of the Xi'an geothermal field, but not from the North Mountains to the north of the field. Based on chemical geothermometers the highest temperature estimated for the Tertiary aquifers of the Xi'an area is around 130 degrees C. (c) 2005 CNR. Published by Elsevier Ltd. All rights reserved.