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Fluids rich in water or carbon dioxide occur only in very small quantities in the interior of the Earth. Despite their low abundance, they may have dramatic effects on the melting temperatures, electrical conductivity and rheology of the mantle. Dissolved volatile components are the driving force behind all explosive volcanic eruptions. Moreover, hydrous fluids are exceptionally good solvents for some trace elements; they can extract these elements from large volumes of ordinary rocks or magmas and deposit them in hydrothermal ore deposits.

Despite their importance for numerous geologic processes, the properties of fluids at elevated pressures and temperatures are generally poorly known. Work at the Bayerisches Geoinstitut during recent years has focussed on the development of new in-situ techniques to study the structure and properties of fluids under extreme conditions. These techniques are based on the combination of externally-heated diamond-anvil cells with powerful infrared and Raman spectrometers. Major progress has been made in understanding the speciation and thermodynamic properties of fluids in the system SiO₂-H₂O as well as in deciphering the phase relationships between hydrous fluids and silicate melts. At the same time, several studies have advanced our understanding of the storage of water in the crystal structure of "normal" mantle minerals. As an example, the environment of hydroxyl groups in MgSiO₃ -ilmenite (akimotoite) was studied in-situ under high pressures in a diamond-anvil cell. Studies of the structure of hydrous silicate melts and glasses have now progressed from very simple model systems to realistic compositions close to natural magmas.