Thermal spring waters and associated gases discharging on several volcanic islands and on mainland Greece along the Hellenic Volcanic Are (HVA) in the south Aegean sea have been investigated. The chemical characteristics of most of the spring waters suggest that the water in their feeding aquifers is largely derived from seawater that enters the hydrological circuits and mixes with local neutral low-salinity bicarbonate waters. There are however discrepancies between the simple theoretical mixing line between local meteoric waters and seawater, and the actual mixing line. This mixing is accompanied by partial Mg and SO4 precipitation to secondary minerals and by the addition of K, Ca, HCO3, B, NH4, and SiO2 to the solutions as a result of water-rock interaction processes. At Miles and Nisyros, where active hydrothermal systems are present at shallow depth, the silica content of the thermal springs is much higher than that of springs on the other islands and mainland Greece. This suggests higher thermal gradients at depth there, and the ascent of geothermal fluids to the surface along fractures prevalently located in the fumarolic fields within the latest eruptive centers and calderas. The fact that the springs are anomalously enriched in silica only at Miles and Nisyros suggests indirectly that, apart from Santorini island, where steam vents are present at the surface, the fluids from the active high-enthalpy geothermal systems do not rise to the surface anywhere else along the HVA. If present, they are well confined under thick and impermeable covers.