Results are presented from a geochemical survey of active submarine warm springs off the island of Vulcano, an active island-arc volcanic centre in the Aeolian Archipelago, Italy. Water samples were collected from submarine ‘fumaroles’ which discharge hot (~ 50–100 °C), acidic, gas-rich, low-chlorinity fluids into the shallow embayment of Porto di Levante, adding dissolved Si, K, Li, Rb, Fe, Mn, NH3 and H2S into surrounding seawater. These fluids are interpreted as a mixture of seawater and low-salinity groundwater which has undergone high-temperature (> 100 °C) hydrothermal alteration, followed by mixing with cool seawater in the sub-seafloor and during venting. The fluid compositions also suggest the chemical ‘overprint’ of reactions resulting from the input of significant amounts of the volcanic gases CO2, SO2 and H2S at this site, specifically, the attack of igneous silicate phases by dissolved CO2, and the hydrolysis and oxidation of SO2 and H2S to SO42−, H+ and elemental sulphur. These overprinting reactions have been proposed for other gas-rich submarine hydrothermal fluids collected from shallow island-arc and hotspot volcanoes, and may be typical of such settings. Several water samples were also collected from a site where warm (< 30 °C) fluids seep from volcanic sand, and are enriched in dissolved Si, K, Li, Rb, Mg, Ca, and particularly Fe and Mn relative to ambient seawater. These solutions are interpreted as the result of low-temperature (< 100 °C) hydrothermal alteration of seawater, again overprinted by the addition of acidic volcanic gases; the warm, acidic fluids then leach Si and metal cations, including Mg2+, from the volcanic sands. The elevated H2S concentrations and low pH of ‘ambient’ embayment water relative to typical surface ocean waters suggest that this type of hydrothermal activity significantly alters the redox and pH conditions of local seawater.