X-ray absorption fine structure (XAFS) spectroscopy was used to measure the coordination structure about Cu2+, Cu1+, and Br- in water at temperatures up to 325 °C. The hexaaqua Cu2+ species maintains its distorted octahedral structure up to 325 °C, whereas at higher temperatures, dehydration reactions occur producing CuO. Under reducing conditions, the dibromo Cu1+ species, [CuBr2]-, is predominant at 200 °C and above for systems having excess Br-. Even for a very high salt concentration of 2.0 m NaBr, only the dibromo Cu1+ species, [CuBr2]-, is observed with no evidence of higher Br- coordination. For this dibromo-species there are no tightly bound hydration waters in the first shell. In the absence of excess Br-, a monoaqua monobromo Cu1+ species, [Cu(H2O)Br] is observed. For certain systems, both Cu and Br XAFS were acquired, and a global model was used to fit the two independent sets of XAFS data. Thus, the results represent a complete picture of the coordination structure about Cu1+ including the coordination numbers, distances for the ion-ion and water-ion associations and also a high-quality measurement of the binding strength and amount of disorder (Debye-Waller factor and the anharmonicity) of the Cu1+/Br- association. Molecular dynamics (MD) simulations were used to further explore the structure and the binding forces for the [CuBr2]- species under hydrothermal conditions. We found quantitative agreement for the Cu-Br interactions, but the simulation has difficulty predicting the experimental Cu-H2O interaction. In particular, the amount of scattering from the water in the dibromo Cu1+ complex was highly over-predicted, so that it is clear that simple intermolecular potential models do not adequately capture this structural feature.