High precision triple oxygen isotope measurements of carbonates can better constrain temperatures and oxygen isotope compositions of seawater through geologic time than ¹⁸O/¹⁶O measurements alone, but lack of a definitive calibration has hindered progress. In this study, we fluorinated both carbonate and water samples to measure quantitatively the triple oxygen isotope composition of each phase. We compared the oxygen isotope fractionation between carbonate and water for different carbonate materials: calcite synthesized with and without carbonic anhydrase, abiogenic calcite from Devils Hole, and extant biogenic calcite and aragonite of marine origin. We found similar 1000lnα¹⁸O_cc-wt values for all materials and combined the results with the high temperature experimental data of O'Neil et al. (1969), resulting in the following fractionation equation (T in Kelvins) 1000lnα18Occ-wt=2.84(±0.02)×106T2-2.96(±0.19). The calcite triple oxygen isotope values yielded a θ-T relationship of θ_cc-wt = –1.39(±0.01)/T + 0.5305 whereas the aragonite triple oxygen isotope values yielded a θ-T relationship of θ_ara-wt = –1.53(±0.02)/T + 0.5305. The calcite-water triple oxygen isotope equilibrium fractionation equation for natural samples is Δ17′Occ-Δ17′Owt=2.84(±0.02)×106T2-2.96(±0.19)-1.39(±0.01)T+0.5305-λ. The combined 1000lnα¹⁸O and 1000lnα¹⁷O relationships can be used to assess equilibrium in ancient samples and to evaluate potential secular changes in the δ¹⁸O value of seawater. Most of the Phanerozoic samples analyzed in this study, which were determined to be pristine in previous studies, have undergone some level of diagenesis. Two samples appear to preserve their original oxygen isotope compositions and suggest a cool ocean with a δ¹⁸O value similar to the modern ocean. Using a fluid-rock interaction model, we can “see through” the diagenetic process and estimate the triple oxygen isotope composition of the carbonate prior to alteration. In doing so, we show that for the time intervals and sample locations measured in this study, Phanerozoic oceans had a comparable range of oxygen isotope compositions and temperatures as modern seawater.