Rare earth element (REE) phosphates such as xenotime (YPO4) are important hosts to the heavy (H)REE in natural systems. Xenotime is commonly associated with hydrothermal alteration and mineral replacement reactions and its composition may yield important clues about the mineralization processes from aqueous fluids in critical mineral deposits. Robust underlying thermodynamic data for the REE phosphate endmembers and aqueous species are required to simulate the stability of xenotime and the mobility of REE in natural hydrothermal fluids. In this study, the solubility of synthetic TbPO4, HoPO4, TmPO4, and LuPO4 endmembers has been measured in aqueous solutions between 100 and 250 ∘C at saturated water vapor pressure. The solubility products (Ks0) determined in the experiments were compared to values retrieved from a combination of calorimetric data for the REE phosphates and thermodynamic properties of the aqueous REE species at elevated temperatures. The solubility of xenotime is retrograde and generally higher in the experiments than predicted by different sources of thermodynamic data. To resolve these discrepancies, the solubility data were used to optimize the thermodynamic properties of the REE phosphate endmembers and REE aqueous species. These optimizations permit retrieving a set of provisional standard Gibbs energies of formation for REE3+ and REEOH2+ at elevated temperature and were used to derive the following updated Ks0 values (uncertainty of ± 0.2 at the 95% confidence) for the reaction REEPO4 (s) = REE3+ + PO43−:Unlabelled Tablet (∘C)logKs0(TbPO4)logKs0(HoPO4)logKs0(TmPO4)logKs0 (LuPO4)100−27.3−27.7−27.9−28.1150−28.8−29.2−29.5−29.6200−30.6−30.9−31.2−31.4250−32.7−32.9−33.3−33.4The updated thermodynamic data generated from the solubility experiments have a significant impact on simulated xenotime compositions and predicted mobility of REE in crustal fluids. Future efforts are necessary to better constrain the properties of REE hydroxyl species at elevated temperature and possible non-ideal solid solution behavior for REE with ionic sizes significantly different from Y3+.