Submarine hydrothermal fluids from JADE and CLAM sites in the mid-Okinawa Trough Backarc Basin are highly enriched in CO2, K, Li, NH4, CH4 and titration alkalinity compared to MOR fluids so far studied, while their H2S and 3He contents are similar. The He and C isotopic ratios as well as these chemical features indicate that the hydrothermal systems are controlled by reaction between seawater and CO2-rich intermediate to acid volcanic rocks of island-arc type, with strong influence of organic matter and are consistent with its initial stage of rifting on a continental plate margin. While the endmember JADE fluid is nearly devoid of Mg and SO4, the CLAM fluid has definite concentrations of Mg (upper limit 22¿5 mM) and, most interestingly, of 34S-enriched SO4 (upper limit 10¿2 mM, Δ34S=27 to 39%).