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The SuSu Knolls and DESMOS hydrothermal fields are located in the back-arc extensional transform zone of the Eastern Manus Basin. In 2006, highly acidic and ΣSO 4 -rich vent fluids were collected at both sites and analyzed for the chemical and isotopic composition of major and trace species. Fluids exiting the seafloor have measured temperatures from 48 to 215 °C and are milky white in appearance due to precipitation of elemental S 0 . Vent fluid concentrations of Na, K, and Mg are depleted by as much as 30% relative to seawater, but have the same relative abundance. In contrast, the fluids are highly enriched in dissolved ΣCO 2 , Cl, SiO 2(aq) , Fe, and Al relative to seawater. Measured pH (25 °C) ranged from 0.95 to 1.87 and aqueous ΣSO 4 ranged from 35 to 135 mmol/kg. The chemical and isotopic composition points to formation via subsurface mixing of seawater with a Na-, K-, Mg-, and Ca-free, volatile-rich magmatic fluid exsolved from subsurface magma bodies during a process analogous to subaerial fumarole discharge. Estimates of the magmatic end-member composition indicate a fluid phase where H 2 O > SO 2 > CO 2 ≈ Cl > F. The hydrogen and oxygen isotopic composition of H 2 O and carbon isotopic composition of ΣCO 2 in the vent fluids strongly suggest a contribution of slab-derived H 2 O and CO 2 to melts generated in the mantle beneath the Eastern Manus volcanic zone. Abundant magmatically-derived SO 2 undergoes disproportionation during cooling in upflow zones and contributes abundant acidity, SO 4 2− , and S 0 to the venting fluids. Interaction of these highly acidic fluids with highly altered mineral assemblages in the upflow zone are responsible for extensive aqueous mobilization of SiO 2(aq), Fe, and Al. Temporal variability in the speciation and abundance of aqueous S species between 1995 and 2006 at the DESMOS vent field suggests an increase in the relative abundance of SO 2 in the magmatic end-member that has mixed with seawater in the subsurface. Results of this study constrain processes responsible for the formation of hot-spring fluids in magmatically active back-arc environments and the resulting chemical exchange between the lithosphere and water column. [ABSTRACT FROM AUTHOR]