In this paper the results of investigations of the mechanism and kinetics of reactions of formation of fine crystalline complicated oxides in thermovaporous and solvothermal conditions are presented. The processes of formation of complicated oxides, magnesian spinel (MgAl2O4), aluminate of lanthanum (LaAlO3), gahnite (ZnAl2O4), barium hexaferrite (BaFe12O19), lithium metaniobate (LiNbO3 and yttrium-aluminum garnet (Y3Al5O12) were studied. Synthesis of complicated oxides carried out both in solvothermal (T = 250-350 degrees C) and in supercritical conditions (T > 380 degrees C) in medium of water fluid. As raw materials were intermixtures of solid simple oxides or hydroxides and, sometimes, oxides with solution of compound containing doping component. It was found that process of complicated oxides formation proceeds into solid phase. In all cases the formation of complicated oxides has gone through a stage of existence of intermediate crystalline substance with Me(OH)n structure. The properties of synthesized complicated oxides mainly are determinated by peculiarities of this intermediate. It is possible to influence on properties of a finished product by changing conditions of formation of intermediate. (c) 2006 Springer Science + Business Media, Inc.