The isotopic and chemical composition of neutral chloride thermal waters (≦100℃) tapped by drillholes at Atagawa, Katase-Shirada, Inatori, Mine and Yazu, Izu p eninsula, Japan, together with neutral sulfate thermal waters (37°～63℃) from Otaki, Yugano, Kochi and Rendaiji hot springs, Izu, have been determined. The isotopic and chemical data suggest that the deep chloride thermal waters are derived from the mixture of local surface water and sea water. Sulfate ion could be derived from the anhydrite deposited in the Yuga-shima formation (common country rock of the thermal waters examined). Sulfate concentration in the thermal waters is limited by the solubility of anhydrite at the bottom of drillholes, except at Mine and Yazu. The behavior of sodium, potassium and calcium ions is also controlled by thermal water-rock equilibria.