Solid state reactions in ceramic materials are investigated since many years under both fundamental and technological points of view. Model experiments are well suited to study various aspects of complex solid state reactions. In this approach, instead of using polycrystalline materials, one reactant is a bulk single crystal. In such model experiments, the formation and orientation of the reaction products can be well characterised by several structural techniques such as X-ray diffractometry (XRD) and transmission electron microscopy (TEM). This Ph.D. work deals with solid state reactions in model experiments. In the work, solid state reactions in different oxide systems, viz. BaCO3-TiO2, BaO-TiO2, SrO-TiO2, CaO-TiO2 and MgO-TiO2, were investigated. The radius of the element reacting with TiO2 was varied from 139 pm for BaO (BaCO3) to 75 pm for MgO. In the first two systems, BaCO3-TiO2 and BaO-TiO2, the solid state reaction of solid BaCO3 and BaO vapour with TiO2 substrates of different crystallographic structure (anatase and rutile) were studied. The main goal of this part of the Ph.D. work was to study the mechanism of BaTiO3 formation in vacuum and in air. In the next three systems, SrO-TiO2, CaO-TiO2 and MgO-TiO2, the solid state reaction between TiO2 (rutile) single crystals and SrO, CaO or MgO vapour was studied. The primary aims of this part of the Ph.D. thesis were: (1) To compare the orientation relationships found for the BaTiO3 perovskite grown on TiO2 (rutile) substrates with those for SrTiO3 and CaTiO3 perovskite as well as for the rhombohedral MgTiO3; (2) To determine possible topotaxial orientation relationships between tetragonal TiO2 (rutile) and the rhombohedral MgTiO3 phase; (3) To study the role of crystallography in topotaxial first phase formation in the MgO-TiO2 system.