This paper reports on investigations of the kinetics and mechanism of the synthesis of simple fine-crystalline oxides from solid-phase precursors in sub- and supercritical water fluid (SCWF). The rate of the processes strongly depends on the water vapor pressure. X-ray analysis, massspectrometry and IR-spectrometry of products obtained during synthesis in water fluid reveal the presence of an intermediate hydrated solid phase. Thus, the synthesis of quartz and corundum in SCWF runs through the stage of the formation of hydrated intermediates. The main type of defects in crystals synthesized in SCWF is oxygen vacancies produced during the dehydroxylation of an intermediate structure when forming the final product. By varying the parameters of the water fluid (P, T), it is possible to influence the local processes of crystal build-up and thus to produce crystals of various size, habitus and degree of perfection.(c) 2007 Elsevier B.V. All rights reserved.