Two distinct hydrothermal aquifers occur beneath the volcanic crater. The deeper one is characterised by temperatures above 330 degrees C and a chemical and isotopic composition in agreement with a thermal modification of seawater by water/rock interaction and boiling processes at depth. The shallow aquifer has temperatures below 170 degrees C and salt content and isotopic composition between those of seawater and local groundwaters. The steam produced at the caldera rim (Kaminakia group) has a delta D and delta(18)O of -42 parts per thousand and -6.1 parts per thousand, respectively, whereas the steam produced inside the caldera (Stephanos and Polybotes group) exhibits average delta values of -11 parts per thousand and +2.5 parts per thousand for D and O-18, respectively. Such values are indicative of different parent waters for the above two aquifers. The isotope geothermometers based on the pairs (CH4-H-2), (H2O-H-2) and (CH4-CO2) from the same fumaroles yield values consistently within the 100-350 degrees C range.