The Puyehue-Cordon Caulle area (40.5degreesS) hosts one of the largest active geothermal systems of Southern Chile, comprising two main thermal foci, Cordon Caulle and Puyehue. Cordon Caulle is a NW-trending volcanic depression dominated by fumaroles at the top (similar to 1500 m) and boiling springs at the northwest end (similar to 1000 m). In the latter, the alkali ne-bicarbonate composition of the springs with low Mg (<0.06 mg/l) relative to the local meteoric waters (similar to5 mg/l), low chloride (<60 mg/l), high silica (up to 400 mg/l) and delta(18)O-deltaD values close to the Global Meteoric Water Line (GMWL), in combination with the large outflow (100 Us), suggest the existence of a secondary steam-heated aquifer overlying a main vapor-dominated system at Cordon Caulle. Subsurface temperatures of the secondary aquifer are estimated to be about 170-180degreesC (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 degreesC and a delta(18)O-deltaD 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 Cordon Caulle and Puyehue represent two separate upflows. (C) 2004 CNR. Published by Elsevier Ltd. All rights reserved.