Abstract An exploration of the Li isotope variations of thermal waters and various rock types from drill cores and outcrops in the Yellowstone National Park, Wyoming-Montana, hydrothermal system was undertaken to investigate simple models for the Li cycle and to consider their implications for the geochemical dynamics of the system. Thermal waters have Li concentrations ranging from 0.27 to 6.5 mg/kg, and δ 7 Li values from 1.0 to 6.5 per mil. Rocks have Li concentrations ranging from 2.0 to 32.7 mg/kg (in sedimentary rock), and 15.2 to 282 mg/kg (in rhyolite); Li concentrations in rhyolite increase with the extent of hydrothermal alteration because Li from thermal water is incorporated in hydrothermal minerals. The range in δ 7 Li values for the sedimentary rock is 1.6 to 22.4, and δ 7 Li is generally higher in limestone than in shale. The range of δ 7 Li values for the rhyolite is −3.6 to +7.5 per mil; δ 7 Li values generally increase with an increasing extent of hydrothermal alteration. Relationships of Li and Cl concentrations in thermal waters indicate a loss of Li by incorporation into hydrothermal minerals in altered rhyolite. Simple isotopic fractionation models for Li isotope exchange between thermal water and altered rhyolite, with mass transfer constraints from Li/Cl ratios of thermal waters, can account for the observed variations in δ 7 Li. Massbalance considerations indicate that the long-term discharge of Li from the Yellowstone hydrothermal system may be sustained in large part by deep input of high-salinity magmatic brine.