Twenty-two cold and thermal waters from shallow wells sampled in June 1995 in the Vulcano Porto area, Vulcano Island, were analyzed for major and minor chemical constituents, oxygen and hydrogen isotopes and tritium contents, and sulfur isotopes in the dissolved sulfate. The sulfur isotopic composition of the dissolved sulfate ranges between +0.6 and +6.5 parts per thousand (mean +3.7+/-1.7 parts per thousand), and is interpreted as deriving mainly from fumarolic SO2 undergoing oxidation in deep and shallow aquifers, with possible minor contributions from oxidation of H2S. Dissolution of secondary anhydrite may have been a minor source of the isotopically heavy aqueous sulfate in the cold groundwaters, The chemical and isotopic features of the waters support previous interpretative hydrologic models of Vulcano Porto, which comprise a number of aquifers fed basically by two major end-members, i.e. meteoric water and crater-type fumarolic inputs, the latter in the form of absorbed emissions or condensate, These data, along with the sulfur isotopes of aqueous sulfate, exclude involvement of seawater in the recharge of the groundwater system of the island. (C) 2001 CNR. Published by Elsevier Science Ltd. All rights reserved.