The parent geothermal water proposed for the Chachimbiro geothermal area has calculated values of 2250 mg/L Cl and approximately 5 bar PCO2. It comes from a reservoir having an estimated temperature of 225-235 degrees C, although temperatures somewhat higher than 260 degrees C may be present at the roots of the system. The geothermal reservoir at Chachimbiro is recharged mainly by meteoric water (about 92%) and secondarily by are-type magmatic water. Carbon and sulfur isotope data support a magmatic origin for the C and S species entering the geothermal system from below, consistent with indications provided by He isotopes. The thermal springs of Na-Cl to Na-Cl-HCO3 type located in the Chachimbiro area originate through dilution of the parent geothermal water and have reached different degrees of re-equilibration with country rocks at lower temperatures. (c) 2005 CNR. Published by Elsevier Ltd. All rights reserved.