The kinetics of the disproportionation reaction of tin(II) oxide (SnO) to tin(IV) oxide (SnO 2 ) and tin metal (Sn) was studied ex-situ and in-situ by X-ray powder diffraction with synchrotron radiation. Depending on sample preparation and decomposition temperature, an intermediate oxide Sn 2 O 3 was observed. From thermal disproportionation at different temperatures, we extracted the reaction rate law and the reaction rate coefficients k and activation energy E A for the disproportionation of SnO and Sn 2 O 3 . The reaction follows a nucleation and growth mechanism according to Avrami-Erofeyev with a reaction exponent m=2. At low temperatures, a synchrotron radiation-induced disproportionation of SnO was observed.