A transport-reaction model calculates isotope ratios (e.g., 87Sr/86Sr) in groundwater systems where isotope ratios in the fluid are affected by two distinct types of water-rock interaction: rapid exchange reactions and slower dissolution reactions. Rapid exchange reactions buffer the groundwater isotope ratios, and a two-stage evolution is observed. After a perturbation in the system an exchange front, which can be described by a retardation equation, propagates through the system at a velocity equal to the groundwater velocity divided by a retardation factor. If steady boundary conditions are maintained, the exchange front is advected out of the system and a quasi-steady state develops in which dissolution reactions influence the isotope ratios but the exchange reactions have no effect. Because clay minerals which readily exchange ions or water molecules with groundwater are found in most hydrologic systems, exchange reactions may significantly affect interpretation of isotopic data.¿ 1997 American Geophysical Union