Hydrothermal synthesis is an important route to novel materials. Hydrothermal chemistry is also an important aspect of geochemistry and a variety of waste remediation technologies. There is a significant lack of information about the speciation of inorganic compounds under hydrothermal conditions. For these reasons we describe a high-temperature, high-pressure cell that allows one to acquire both x-ray absorption fine structure ͑XAFS͒ spectra and x-ray transmission and absorption images of

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... ogeneous hydrothermal mixtures. We demonstrate the utility of the method by measuring the Cu͑I͒ speciation in a solution containing both solid and dissolved Cu phases at temperatures up to 325°C. X-ray imaging of the various hydrothermal phases allows micro-XAFS to be collected from different phases within the heterogeneous mixture. The complete structural characterization of a soluble bichloro-cuprous species was determined. In situ XAFS measurements were used to define the oxidation state and the first-shell coordination structure. The Cu-Cl distance was determined to be 2.12 Å for the CuCl 2 Ϫ species and the complete loss of tightly bound waters of hydration in the first shell was observed. The microreactor cell described here can be used to test thermodynamic models of solubility and redox chemistry of a variety of different hydrothermal mixtures.