Spectroscopic studies can provide information on a large variety of physical and chemical processes in solids. In this contribution, we report on studies of homogeneous, and heterogeneous solid state reactions as well as on studies of diffusion processes in complex oxides. Examples have been drawn from work performed by means of optical spectroscopy and Mossbauer spectroscopy under in in situ conditions at high temperatures and at defined oxygen partial pressures. Examples for homogeneous reactions refer to order-disorder processes and their kinetics in nickel aluminate spinel and magnetite. Studies of heterogeneous solid-solid and solid-gas reactions relate to the formation of cobalt aluminate spinel and to redox processes in fayalite, Fe2SiO4. respectively. The diffusion of the iron cations in Fe2SiO4, has been observed by means of Mossbauer spectroscopy. Finally, an example is given of the application of Mossbauer spectroscopy for providing insight into mechanically induced reactions (mechanochemistry) in a spinel.