Chemical analyses of thermal springs and gas emissions on the volcanic island of Pantelleria, southern Italy, suggest the presence, at depth, of a NaCl thermal aquifer near 100??C; 80-90% of the water in this aquifer is fed by rainfall and the remainder of the mixture is seawater. Chemical analyses of the gas emissions reveal that below this shallow aquifer, a deeper hydrothermal degassing system (>150??C) may exist. Rising CO2 causes alterations of the upper aquifer host rocks (mostly pantellerites), favors formation of NaHCO3 type waters, and causes eventual rpecipitation of CaCO3 and coprecipitation of MgCO3 and silica. Comparison of the present chemical data with older data from the literature suggests that the shallow Pantelleria thermal aquifer has increased in salinity and decreased in temperature during the last 25 a. Both effects were apparently caused by increased flow of recent seawater into the system because of increased ground water abstraction in the northern part of the island. ?? 1994.