Two distinct hydrothermal aquifers occur beneath the volcanic crater[ The deeper one is characterised by temperatures above 229>C and a chemical and isotopic composition in agree! ment with a thermal modi_cation of seawater by water:rock interaction and boiling processes at depth[ The shallow aquifer has temperatures below 069>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 dD and d 07 O of -31-and -5[0-\ respectively\ whereas the steam produced inside the caldera "Stephanos and Polybotes group# exhibits average d values of -00-and ยฆ1[4-for D and 07 O\ respectively[ Such values are indicative of di}erent parent waters for the above two aquifers[ The isotope geothermometers based on the pairs "CH 3 รH 1 #\ "H 1 OรH 1 # and "CH 3 รCO 1 # from the same fumaroles yield values consistently within the 099ร249>C range[ ร 0888 CNR[ Published by Elsevier Science Ltd[ All rights reserved[ Keywords] volcanic arc^water chemistry^gas chemistry^water isotopes^steam^Nisyros^Greece 0[ Introduction Nisyros is located some 09 km from the Turkish coast and belongs to the Dode! canese group of islands in the Aegean Sea[ It is the easternmost island of the Aegean