AbstractThe reactions of the oxides SiO2, MoO3, and WO3with steam at temperatures between 400 and 700 °C and pressures of 5 to 500 atm were studied by means of the transfer method. The results were evaluated by a new method. In the system Sio2/H2O the formation of gaseous Si(OH)4, Si2O(OH)6, and [SiO2(OH)2]xwas found to occur in three different water‐density ranges. In the systems MoO3/H2O and WO3/H2O the gaseous compounds MoO2(OH)2and WO2(OH)2, which were already known, exist up to densities of approximately 0.05g/cm3. At higher densities, at which the supercritical phase begins to show the properties of a liquid to an ever increasing extent, isopolymolybdic or isopolytungstic acids appe