δ18O values of the Heavitree Quartzite, central Australia, vary systematically as a function of deformation and recrystallization in a duplex composed of 5 superposed thrust sheets which formed under greenschist-grade conditions. Undeformed quartzite of the basal thrust sheet 1 and moderately strained and partially recrystallized quartzite of thrust sheet 2 range over 4%. in δ18O values and have an average value of ~12.8%., Entirely recrystallized quartzite of overlying thrust sheet 3 retains a similar average value of ~12.3%. but shows a narrow range of ~1.5%., suggesting isotopic homogenization. Recrystallized quartzites of sheet 5 have a lower δ18O value than the other thrust sheets. The homogenized δ18O values of sheet 3 relative to sheets 1 and 2 could not have resulted from mechanical mixing, isotopic exchange during dissolution-reprecipitation, or exchange by diffusion between the quartzite and a fluid in microfractures or along grain boundaries. The homogenized δ18O values of sheet 3 and lowered values of sheet 5 are interpreted to have been produced by isotopic exchange during recrystallization by grain-boundary migration. This is probably an important mechanism of isotopic exchange for rocks undergoing dynamic recrystallization.