Using a variety of chemical geothermometers and statistical analysis, we estimate the temperature of a possible deeper geothermal reservoir at Bugok, Southern Korea. Shallow thermal aquifers (down to about 400 to depth) are under exploitation in this area; the temperatures (up to 78 degrees C) of the produced fluids are the highest found in South Korea. Based on hydrochemical data and occurrence, the groundwaters at Bugok can be classified under three groups: Na-SO4 thermal groundwaters (CTGW) occurring in the central (about 0.24 km(2)) part of the area; Ca-HCO3 cold groundwater (SCGW) found in shallow peripheral parts of the CTGW; the intermediate-type groundwater (STGW). The CTGW type is typical of the Bugok thermal waters; they have the highest discharge temperatures and contain very high concentrations of Na (75.1-101.0 mg/L), K (2.9-6.9 mg/L) and SiO2 (62.0-84.5 mg/L) and are rich in sulfates. The major ion composition of the CTGW suggests that these waters are in partial equilibrium with rocks at depth. The application of various alkali-ion geothermometers yields temperature estimates in the 88-198 degrees C range for the thermal reservoir. Multiple-mineral equilibrium calculations indicate a similar but narrower temperature range (from about 100 to 155 degrees C). These estimates for CTGW are significantly higher than the measured discharge temperatures. Considering the heat losses occurring during the ascent of the waters, one can infer the presence of a deeper (around 1.8 km) thermal reservoir in the Bugok area that could be developed for district heating or other direct applications of geothermal heat. (c) 2006 CNR. Published by Elsevier Ltd. All rights reserved.