A study was conducted on the transformation of SnO to SnO2 using X-ray diffraction and subjecting the SnO to heat treatments between 300 °C < T < 600 °C in two different atmospheres, argon and air. The intermediary oxide that appears in the disproportionation process was identified as Sn2O3. In an argon atmosphere, decomposition occurs in three stages: (1) a direct transformation of SnO to SnO2, (2) the formation of some intermediary Sn2O3 from SnO, and (3) the conversion of the Sn2O3 to SnO2 with the formation of metallic tin, Sn (l). When an atmosphere of air is used, however, a reaction occurs, concurrent with the decomposition reactions, that relates to the specific oxidation of the metallic tin produced in the course of the three process stages.