Fluid inclusions in barite and sulfide in chimneys (both active and inactive) from three hydrothermal sites of the back arc Lau basin were studied with microthermometric and isotopic methods to determine the chemistry and evolution of hydrothermal aqueous fluids. Strontium isotope compositions of sulfides from the Lau basin reflect the presence of anhydrite and barite inclusions. The 87Sr/86Sr ratios of these two sulfate minerals vary from 0.7045 to 0.7078 and are interpreted as the result of mixing between various proportions of the hydrothermal end-member and pure seawater. The microthermometric study of fluid inclusions reveals that mixing with seawater involved different kinds of aqueous fluid end-members. A high-temperature Mg-depleted end-member of high salinity (≥5.5 wt % eq NaCl) was found at the Vai Lili site. A uncommon low-temperature Mg-rich end-member was also identified at the Hine Hina site in association with barite deposition. At the Vai Lili site, very low salinity fluids were produced in addition to a very saline brine (≥30 wt % NaCl) that was trapped inside anhydrite precipitated from an active vent at a temperature of 342 ¿C. Oxygen isotope ratios of water inclusions range from 2? to 4.4? for chalcopyrite, barite, and sphalerite minerals. The 18O enrichment and the high salinities of many fluids from the Lau basin are accounted for by the specificity of the back arc setting. This non mid-ocean ridge setting is characterized by the shallow depth of the hydrothermal systems that allows frequent unmixing of high-temperature liquids. The abundance of silicic magmas also provided magmatic fluids (including brines) that mixed with seawater-derived aqueous fluids. ¿ 1999 American Geophysical Union