The Miravalles high-temperature geothermal reservoir, located in the northwestern part of Costa Rica, is liquid-dominated. Reservoir temperatures generally range between 230 and 240 degreesC, The highest measured value is 255 degreesC. Bottom-hole measurements and solute geothermometry indicate that thermal conditions within the reservoir are very stable over time. The waters discharged from the wells have a neutral or slightly alkaline pH and are of the sodium-chloride type. Based on isotope data, the main recharge zone appears to be located on the northeastern side of the Guanacaste Cordillera. Several mixing trends have been identified between reservoir fluids and regional groundwaters. Gas discharges are dominated by CO2, with minor amounts of H2S and N-2. Relative N-2, Ar and He contents reveal a typical arc-type signature and significant inflow of meteoric-derived gases. Cl SiO2-enthalpy and delta(18)O delta(2)H-Cl relationships suggest the existence of a maturation trend that is the result of both natural (i.e. direct drainage of deeper fluids) and anthropogenic causes (reinjection of Cl-rich waste waters). Acid fluids with SO4-acidity (pH ranging between 2.4 and 3.7) have been encountered in three wells at the eastern border of the well field. Preliminary data assessment indicates two possible sources, either superficial H2S oxidation or inflow of "immature" volcanic waters. (C) 2002 CNR. Published by Elsevier Science Ltd. All rights reserdved.