Hydrogen and oxygen isotope ratios of thermal waters from 42 hot springs in Izu Peninsula, Shizuoka Prefecture, range from -51.7 to -24.7% in δD and from -7.9 to -4.1% in δ18O, respectively. The isotope ratios suggest that most of the thermal waters in Izu Peninsula are essentially meteoric in origin. Significantly deviated from the normal meteoric waters are thermal waters from Yumigahama, Kumomi, and Shimogamo hot springs which may be mixtures of seawater and local meteoric waters, and thermal waters from Izusan and Imaihama whose oxygen isotope ratios may have become heavier by water-rock interaction.
