The dramatic change in properties of water near its critical point (i.e. T = 374 °C and p = 22.1 MPa, note: 100 MPa = 0.1 GPa = 1 kbar ≈ 1000 atm) has been a subject of numerous studies and also lead to the development of various applications (e.g. in waste destruction, biomass processing, and the synthesis of advanced ceramic materials). However, comparatively little is known about the behavior of water at gigapascal pressures. The present study attempts to explore catalytical properties and reactivity of extreme water with respect to several oxide systems: SiO2, TiO2 and LiAlSiO4. “Extreme water” here is defined as existing at p,T conditions of 0.25–10 GPa and 200–1000 °C, thus considering both supercritical fluid and hot compressed ice. The study shows that extreme water can make high pressure mineral phases accessible at relatively mild T conditions. At the same time, high pressure aqueous environments appear efficient in stabilizing novel metastable structures and may be considered as a general route for synthesizing new materials. The hydrothermal treatment of SiO2 glass at 10 GPa and 300–550 °C yielded an unusual ultrahydrous form of stishovite with up to 3% of structural water. At the same time, the extreme water environment enhanced notably the kinetics of stishovite formation, making it accessible at unprecedentedly low temperatures. Thus, for the SiO2–H2O system water acts as both catalyst and reactant. For TiO2 a hydrothermal high pressure treatment proved to be of high importance for overcoming the kinetical hindrance of the rutile – TiO2-II transformation. 6 GPa and 650 °C were established as the mildest conditions for synthesizing pure TiO2-II phase in less than two hours. The crystallization of LiAlSiO4 glass in an extreme water environment yielded a number of different phases. In the low pressure region (0.25 – 2 GPa) mainly a zeolite (Li-ABW) and a dense anhydrous aluminosilicate (α-eucryptite) were obtained. At pressures above 5 GPa the formation of novel pyroxene-like structures with crystallographic amounts of structural water was observed. The overall conclusion of this study is that extreme water environments show a great potential for catalyzing phase transitions in oxide systems and for stabilizing novel structures via structural water incorporation. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>