The composition of metamorphic solutions can be estimated from mineral assemblages, provided the appropriate reaction partners are present and the necessary laboratory calibrations are available. However, ƒ H 2O buffers or indicators are not common and this represents a severe handicap, since solubilities and dissociation equilibria depend strongly on the density of the aqueous fluid, ϱ H 2O . It might be possible in the future to estimate ƒ O 2 and then measure the intrinsic hydrogen fugacity of natural assemblages. Mineral growth and dissolution can be understood only if the necessary information is available on solute speciation. Dissociation constants are available for H 2O, NaCl, KCl, MgCl 2, HCl, but little information is available on speciation in H 2OCO 2 fluids. A review of solubility data in mineral-H 2O, mineral-H 2OHCl and mineral-H 2OCO 2 systems reveals that data are available for surprisingly few minerals and this hampers our ability to predict solute concentrations and hence compositional gradients in the supercritical fluid. Quartz, albite, magnetite, Mg-silicates and calcite are used as examples. Experimental techniques for studying mineral reactions in supercritical mixtures include a variety of fugacity buffers, all based on hydrogen diffusion and two types of hydrogen sensors, one based on the AgAgCl buffer and the other on a modified Shaw bomb. Two examples of equilibrium applications are discussed. The first is concerned with properties of KClH 2O and NaClH 2O mixtures and the second with speciation in an aqueous solution in equilibrium with K-feldspar + muscovite + quartz. Chloride concentration and temperature are shown to affect dissociation equilibria and species distribution most. Metasomatism can occur either by diffusion or fluid flow or both. A sufficiently detailed theoretical framework is available to model diffusional transport, but fluid flow and hence infiltration metasomatism are still not well understood. The effect of combining convecting fluids with chemical reactions can be illustrated in processes of ore formation such as magnetite and sulfide deposits. We can best summarize the state of the art by listing a number of key areas where progress is essential for a better understanding of metamorphic fluids.