High deposition or annealing temperatures can lead to solid state reactions between different phases or between thin films or nanostructures and the substrate. Investigations of phase formation processes during thin-film solid state reactions are thus of significant value. In particular, interrelations between the microstructure of the reaction front, interfacial reaction mechanisms, and the kinetics of the reaction are studied. Topotaxial, i.e. crystallographically well-defined model systems, and systems of practical interest are under consideration.