- Diffusion models predict that peak metamor-phic temperatures are best recorded by the oxygen iso-tope 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 ther-mometry from veins in high-grade metamorphic pelites gives temperatures of 635–690 °C. These temperatures are in excellent agreement with independent thermome-try 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 tempera-tures (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 poly-phase host-rock system. The data demonstrate that bimineralic vein assemblages can be used as accu-rate thermometers in high-temperature rocks whereas retrograde exchange remains a major problem in many polymineralic rocks.