The composition of the supercritical fluid phase which is in equilibrium with metamorphic or igneous mineral assemblages can be assessed either through fluid inclusion studies (Touret, 1971; Roedder, 1972; Poty et al 1974), or by considering the nature and composition of the minerals themselves. In this review we restrict ourselves to the latter approach. It is based on a combination of buffer and exchange reactions and fugacity indicators in conjunction with laboratory calibrations. A simple buffer reaction is a reaction between a group of solids and a single gas species. When all participants in the reaction are present, the chemical potential of the gas species remains constant at a given P and T, regardless of any changes in bulk composition, provided the compositions of the solids remain constant. Hematite + magnetite, quartz + calcite + wollastonite, gypsum + anhydrite, pyrite + pyrrhotite, are commonly occurring buffers for oxygen, CO2, H2O and S2 respectively.