The chemical composition and D/H, 18O 16O and 34S 32S ratios have been determined for the acid hot waters and volcanic gases discharging from Zaō volcano in Japan. The thermal springs in Zaō volcano issue acid sulfate-chloride type waters (Zaō) and acid sulfate type waters (Kamoshika). Gases emitted at Kamoshika fumaroles are rich in CO 2, SO 2 and N 2, exclusive of H 2O. Chloride concentrations and oxygen isotope data indicate that the Zaō thermal waters issue a fluid mixture from an acid thermal reservoir and meteoric waters from shallow aquifers. The waters in the Zaō volcanic system have slight isotopic shifts from the respective local meteoric values. The isotopic evidence indicates that most of the water in the system is meteoric in origin. Sulfates in Zaō acid sulfate-chloride waters with δ 34S values of around +15‰, are enriched in 34S compared to Zaō H 2S, while the acid sulfate waters at Kamoshika contain supergene light sulfate ( δ 34 S = ∼ + 4‰ ) derived from volcanic sulfur dioxide from the volcanic exhalations. The sulfur species in Zaō acid waters are lighter in δ 34S than those of other volcanic areas, reflecting the difference in total pressure.