- Diffusion models predict that peak metamorphic temperatures are best recorded by the oxygen isotope fractionation between minerals in a bi-mineralic rock in which a refractory accessory mineral with slow oxygen diffusion rate is modally minor to a mineral with a faster diffusion rate. This premise is demonstrated for high-grade metamorphism on the island of Naxos, Greece, where quartz–kyanite oxygen isotope thermometry from veins in high-grade metamorphic pelites gives temperatures of 635–690 °C. These temperatures are in excellent agreement with independent thermometry for the regional M2 peak metamorphic conditions and show that the vein minerals isotopically equilibrated at the peak of metamorphism. Quartz–sillimanite fractionations in the same veins give similar temperatures (680±35 °C) and suggest that the veins grew near to the kyanite–sillimanite boundary, corresponding to pressures of 6.5 to 7.5 kbar for temperatures of 635–685 °C. By contrast, quartz–kyanite and quartz–biotite pairs in the host rocks yield lower temperature estimates than the veins (590–600 and 350–550 °C, respectively). These lower apparent temperatures are also predicted from calculations of diffusional resetting in the polyphase host-rock system. The data demonstrate that bimineralic vein assemblages can be used as accurate thermometers in high-temperature rocks whereas retrograde exchange remains a major problem in many polymineralic rocks.