The Puyehue-Cordón Caulle area (40.5◦S) hosts one of the largest active geothermal systems of Southern Chile, comprising two main thermal foci, Cordón Caulle and Puyehue. Cordón Caulle is a NW-trending volcanic depression dominated by fumaroles at the top (∼1500 m) and boiling springs at the northwest end (∼1000 m). In the latter, the alkaline-bicarbonate composition of the springs with low Mg (<0.06 mg/l) relative to the local meteoric waters (∼5 mg/l), low chloride (<60 mg/l), high silica (up to 400 mg/l) and 18O– D values close to the Global Meteoric Water Line (GMWL), in combination with the large outflow (100 l/s), suggest the existence of a secondary steam-heated aquifer overlying a main vapor-dominated system at Cordón Caulle. Subsurface temperatures of the secondary aquifer are estimated to be about 170–180 ◦C (corrected silica geothermometers). The Puyehue thermal area, on the other hand, includes Mg-rich hot springs discharging along stream valleys, with maximum temperatures of 65 ◦C and a 18O– D signature resembling the local meteoric composition, which suggests that the surface manifestations contain a reservoir component that is strongly diluted by meteoric waters. Topographic/hydrologic and chemical characteristics suggest that Cordón Caulle and Puyehue represent two separate upflows.