Stable-isotope analyses were carried out on hydrothermal minerals sampled from the deep metamorphic units at Larderello, Italy. The partial derivative O-18 obtained for the most retentive minerals, quartz and tourmaline, are from +12.0 parts per thousand to + 14.7 parts per thousand and 9.9 parts per thousand, respectively, and indicate deposition from an O-18-rich fluid. Calculated partial derivative O-18 values for these fluids range from + 5.3 parts per thousand to + 13.4 parts per thousand. These values, combined with available fluid inclusion and petrographic data, are consistent with the proposed existence of an early thermal fluid of probable magmatic origin and a late meteoric water. Mixing between these two fluids occurred locally.