Our current understanding of metal speciation in natural hydrothermai systems is based largely on experimental data derived from relatively simple H20-salt systems. However, complexing in natural multicomponent systems may be influenced by additional fluid components that are not considered in this experimental data base. Direct spectroscopic analyses of aqueous species in fluid inclusions at elevated temperatures may provide the most accurate representation of metal complexing in palaeohydrothermal solutions. However, it has been suggested that, in some cases, aqueous species in fluid inclusions may be influenced by the loss or addition of hydrogen by re-equilibration with an ambient fluid reservior (Mavrogenes and Bodnar, 1994; Sterner et al., 1995). In this study, the possible influence of hydrogen diffusion on zinc complexing in high salinity fluid inclusions from polymetallic sulphide quartz veins was investigated by analysing X-ray absorption fine structure (XAFS) spectra obtained from individual fluid inclusions at temperatures up to 400~ before and after experimental re-equilibration at high hydrogen pressures.