The Upper Tigris River Basin is one of the biggest basins in Turkey, where municipal, agricultural and industrial water supplies are highly dependent on groundwater and surface water resources. The interpretation of plots for different major ions indicates that the chemical compositions of the surface/groundwater in the Upper Tigris River Basin are dominated Ca2+, Mg2+, HCO3- and SO42- which have been arisen largely from chemical weathering of carbonate and evaporate rock, and reverse ion exchange reactions. Isotopic composition of surface and groundwater samples is influenced by two main air mass trajectories: one originating from the Central Anatolia that is cold and rainy and another originating from the rains falling over northeastern Syria that is warm and rainy, with warm winds. The relative abundance of cations and anions in water samples is in the order: Ca2+>Mg2+>Na+>K+ for cations and HCO >Cl->SO42-, respectively. Majority of the water samples are plotted on a Piper diagram showing that the chemical composition of the water samples was predominantly Ca-Mg-HCO3 type. Groundwater and surface water have an average (Ca2++Mg2+/2HCO(3)(-)) ratio of 0.65 and 0.74, indicating no significant difference in their relative solute distribution and dissolution of carbonate rock (calcite and dolomite) predominantly by carbonic acid. The Mg2+/Ca2+ and Mg2+/ HCO3- molar ratio values are ranging from 0.21 to 1.30 and 0.11 to 0.47 for the groundwater and from 0.13 to 2.46 and 0.10 to 0.61 for the surface water samples, respectively,indicatingthat significant contribution of dolomitedissolution has a higher advantage over limestone within the Upper Tigris River Basin.