The Vulsini Mts. volcanic region (central Italy) has been studied extensively from geological, petrological and volcanological points of view (e.g. Funiciello et alii, 1977; Barton et alii, 1982; Holm et alii, 1982; De Rita et alii, 1983; Barberi et alii, 1984; Pec-Cerillo, 1985; Peccerillo et alii, 1987; Nappi et alii, 1987). The area has suffered the Apennine orogeny in the Tortonian. During the compressional phase the allochthonous geological units (Ligurides) were thrust upon the "Tuscan and Umbrian Series", the latter forming the backbone of the Apennine belt. During the "late Miocene-Pliocene" an extensional tectonic regime originated several NNW-SSE oriented marine and continental sedimentary basins that characterise central Italy (e.g. Abbate et alii, 1970; Dalian Nardi & Nardi, 1972; Boccaletti et alii, 1980; La Vecchia & Stoppa, 1989). The magmatic events that occurred between 0.85 and 0.06 Ma (Funiciello et alii, 1977; Nicoletti et alii, 1979, 1981; De Rita et alii, 1983) mainly consisting of potassic and high potassic volcanic products (e.g. Conticelli et alii, 1986, 1987) can be referred to this extensional phase (e.g. Amodio et alii, 1987). An anomalous heat flow developed as a consequence of this magmatic activity (e.g. Calamai et alii, 1976), producing two geothermal fields at depth (Latera and Torre Alfina) discovered in the past 25 years. A geochemical survey on gas (16 samples) and water (95 samples) discharges from the Mts. Vulsini volcanic area was carried out in order both to understand their genesis and evolution and to propose a theoretical model for fluid circulation. The meteoric waters, infiltrating from permeable terrains, feed aquifers located at different depths; in the volcanic rocks at least two main reservoirs have been recognised. The shallower one consists of relatively cold and low salinity waters that, in the hydrological basin of Bolsena Lake, assumes a centripetal circulation towards the lake, although in the southernmost part the main water flow is mainly directed towards its outflow (Marta River). The deeper aquifer, confined between the impermeable formations (Ligurides and Neogene marine sediments) underlying the volcanic products and the less permeable volcanic rocks, is represented by relatively high salinity thermal waters uprising through the main fault systems along with deep-originated gas phases (mainly CO <sub>2</sub> ). These fluids tend to affect the shallow aquifer close to their discharge areas mainly in terms of temperature and salinity.