This proposal aims at improving the current understanding of lead (Pb) speciation in hydrothermal fluids using high resolution X-ray absorption spectroscopy (HERFD-XAS). In nature, Pb generally occurs in association with highly sought-after Cu and Zn. As the future supply of Cu and Zn is influenced not only by resource availability but also by the social and environmental sustainability of mining practice, understanding Pb’s potential to act as an environmental contaminant is crucial. Specifically, determining the conditions under which Pb is least mobile is essential. To refine our knowledge on Pb mobilization and complexation with Cl, S, and H2O we propose to conduct in situ HERFD-XAS measurements on hydrothermal fluids up to 600 °C and pressures of 60 MPa at the FAME UHD BM16 beamline. XAS, , autoclave, h,