Abstract Isotopes are forms of an element with the same number of electrons and protons but a different number of neutrns and therefore different masses. Although chemical behavior differs very little between isotopes of the same element, the mass differences between isotopes do produce small chemical differences and equilibrium constants (fractionation factors) for isotopic exchanges are typically close to one. The relative abundances of isotopes commonly analyzed in geothermal fluids were given by Panichi and Gonfiantini (1976). Results of the application of isotopes to geothermal systems have been reviewed by Craig (1963), Panichi and Gonfiantini (1976), Truesdell and Hu1ston (1980), and Giggenbach et al. (1983).