The transport and deposition of copper in saline hydrothermal fluids are controlled by the stability of copper(I) complexes with ligands such as chloride. Despite their role in the formation of most hydrothermal copper deposits, the nature and stability of Cu(I) chloride complexes in highly saline brines remains controversial. We present new X-ray absorption data (P = 600 bar, T = 25-400 °C, salinity up to 17.2 m Cl), which indicate that the linear CuClx 1 - x (x = 1, 2) complexes are stable up to supercritical conditions. Distorted trigonal planar CuCl3 2 - complexes predominate at room temperature and at high salinity (>3 m LiCl): subtle changes in the XANES spectrum with increasing salinity may reflect geometric distortions of this CuCl3 2 - complex. Similar changes were observed in UV-Vis data [Liu, W., Brugger, J., McPhail, D.C., Spiccia, L., 2002. A spectrophotometric study of aqueous copper(I) chloride complexes in LiCl solutions between 100 °C and 250 °C. Geochim. Cosmochim. Acta 66, 3615-3633], and were erroneously interpreted as a new species, CuCl4 2 -. Our XAS data and ab-initio XANES calculations show that this tetrahedral species is not present to any significant degree in our solutions. The stability of the CuCl3 2 - complexe decreases with increasing temperature; under supercritical conditions and in brines under magmatic-hydrothermal conditions (e.g., 15.58 m Cl, 400 °C, 600 bar), only the linear Cu(I) chloride complexes were observed. This result and the instability of the CuCl4 2 - complex are also consistent with the recent ab-initio molecular dynamic calculations of Sherman [Sherman D. M.(2007) Complexation of Cu+ in hydrothermal NaCl brines: ab-initio molecular dynamics and energetics. Geochim. Cosmochim. Acta 71, 714-722]. This study illustrates the power of the quantitative nature of XANES and EXAFS measurements for deciphering the speciation of weak transition metal complexes up to magmatic-hydrothermal conditions. The systematic XANES data are used to retrieve the formation constant for CuCl3 2 - at 150 °C, which is in good agreement with the reinterpretation of the UV-Vis data of Liu et al. (Liu et al., 2002). At high temperatures (≫400 °C), the solubility of chalcopyrite in equilibrium with hematite-magnetite-pyrite and K-feldspar-muscovite-quartz calculated with the new properties is lower than that calculated using the previous model, and the calculated solubilities are at the lower end of the range of values measured in brine inclusions from porphyry copper systems.