The phase stability, nonstoichiometry and point defect chemistry of polycrystalline Sr2FeMoO6-δ (SFMO) was studied by thermogravimety at 1000, 1100, and 1200 °C. Single-phase SFMO exists between -10.2≤log pO2≤-13.7 at 1200 °C. At lower oxygen partial pressure a mass loss signals reductive decomposition. At higher pO2 a mass gain indicates oxidative decomposition into SrMoO4 and SrFeO3-x. The nonstoichiometry δ at 1000, 1100, and 1200 °C was determined as function of pO2. SFMO is almost stoichiometric at the upper phase boundary (e.g. δ=0.006 at 1200 °C and log pO2=-10.2) and becomes more defective with decreasing oxygen partial pressure (e.g. δ=0.085 at 1200 °C and log pO2=-13.5). Oxygen vacancies are shown to represent majority defects. From the temperature dependence of the oxygen vacancy concentration the defect formation enthalpy was estimated (∆HOV=253±8 kJ/mol). Samples of different nonstoichiometry δ were prepared by quenching from 1200 °C at various pO2. An increase of the unit cell volume with increasing defect concentration δ was found. The saturation magnetization is reduced with increasing nonstoichiometry δ. This demonstrates that in addition to Fe/Mo site disorder, oxygen nonstoichiometry is another source of reduced magnetization values.