Recent studies provide theoretical models for kinetically limited isotope exchange during fluid flow; however, few have attempted to apply these models to natural systems. In this paper, we interpret stable isotopic data from fossil hydrothermal systems in two different tectonic settings by using combined equations of mass transport and kinetically limited isotope exchange. Our models provide alternative interpretations for the geometry of flow responsible for the development of 18O depletion annuli surrounding epizonal intrusions and for the geometry of flow in shallow-level normal faults. Our models also provide information about relative isotope exchange rates and, by extension, mechanisms of isotope exchange. Our results indicate that dynamic recrystallization in fault zones promotes exchange by surface reaction, whereas exchange in chemically nonreactive and undeformed minerals may be limited by solid-state diffusion.