The Mount Sabalan district is regarded as the best place to investigate geothermal activities in northwest Iran. Since the last episode of volcanic activity in the Plio-Quaternary time, hot springs and surficial steams as conspicuous manifestation of geothermal activities have appeared around the slopes of Mount Sabalan. The hot fluids circulating in this geothermal field contains anions chiefly of HCO3- and Cl-; however, SO42- content in some water samples is relatively high, imparting sulfate characteristics to such fluids. Geothermometric studies provided compelling evidence for estimation of the reservoir temperature (~150 °C) in the study areas. Thus, in this respect, the geothermal systems in the east of Mount Sabalan were categorized as high-temperature. The composition of stable isotopes of oxygen (?O) and hydrogen (δD) indicated that the waters involved in this geothermal field have mainly meteoric origin. On the basis of 3H isotopes, only a few water samples exhibited a residence time of ~63 years, which can be grouped as old waters.