The island of Kos is located in the southern part of the Aegean volcanic are. Between 1991 and 1993, water and gas samples were collected from cold and thermal springs and from several water drill-holes. The maximum surface temperature of 45 degrees C is associated with Fokas spring water, which consists of a seawater modified by interaction with sedimentary rocks in the NW parr of the island. Two distinct groundwater circulations develop in the sedimentary and volcanic rocks; these are meteoric waters dominated by HCO3 ions that have ions formed by interaction with the calcite present in the rocks as primary or secondary mineral. Addition of CO2 from an external source to the groundwaters locally leads to the formation of aggressive waters and a water-rock interaction characterised by the leaching of CaSO4 and partial attainment of mineral/fluid equilibrium. The gas compositions suggest the presence, at depth, of a CO2-producing high-temperature system in its waning stage. an the basis of chemical solute geothermometry, temperatures higher than 110 degrees C appear unlikely at relatively shallow depths.