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Measurements of oxygen isotope fractionations between coexisting igneous titanite (Ttn) and zircon (Zrc) have been used to formulate a self-consistent, empirical calibration of equilibrium oxygen isotope partitioning: 1000 lnα(Zrc−Ttn)=1.02×106/(T2)This calibration is based on the average measured Δ(Zrc-Igneous Ttn) = 1.2 ± 0.3‰ (n= 27 rocks) and a closure temperature of titanite to oxygen diffusion of approximately 650°C. The average measured fractionation between zircon and metamorphic titanite is 2.1 ± 0.4‰ (n= 5 rocks). These results show that Δ(Zrc-Ttn) can be used to distinguish igneous vs. metamorphic (or hydrothermal) titanite.