Speciation of metal cations in hydrothermal fluids influences key processes such as transport and mobilization of these metals in hydrothermal settings or fractionation between melt, fluid and vapor. We have used First-Principle Molecular Dynamics (FPMD) simulations to study the speciation of coinage metals as well as platinum group elements in hydrothermal fluids of various compositions. In particular, we have investigated the speciation of gold and platinum with sulfur species including the radical S3-•. Structures obtained from FPMD were used to compute XAS spectra for comparison with experiment, allowing identification of the metal complexes. Furthermore, in this talk, we will show how to compute equilibrium constants between different complexes from FPMD simulations. By computing the equilibrium constant for ligand exchange between HS- and S3-• we were able using thermodynamical modelling to reproduce the high solubility experimentally measured of these metals in sulfur rich solutions.