Three types of solid state reactions - namely the precipitation formation (dissolution) of spinel or metal particles, both in (A(1-x)B(x))(1-delta)O solid solutions, and the oxidation of metal foils to the corresponding oxide BOn - were studied first by means of DEXAFS spectroscopy. This technique allows us to follow the different solid state reactions in situ and time resolved, after changing the oxygen activity at elevated temperature (770 < T < 1450 K).