We characterize experimentally and theoretically the promising new solid oxide fuel cell electrode material Sr2Fe1.5Mo0.5O6-delta (SFMO). Rietveld refinement of powder neutron diffraction data has determined that the crystal structure of this material is distorted from the ideal cubic simple perovskite, instead belonging to the orthorhombic space group Prima. The refinement revealed the presence of oxygen vacancies in the as-synthesized material, resulting in a composition of Sr2Fe1.5Mo0.5O5.90(2) (delta = 0.10(2)). DFT+U theory predicts essentially the same concentration of oxygen vacancies. Theoretical analysis of the electronic structure allows us to elucidate the origin of this nonstoichiometry and the attendant mixed ion-electron conductor character so important for intermediate temperature fuel cell operation. The ease with which SFMO forms oxygen vacancies and allows for facile bulk oxide ion diffusivity is directly related to a strong hybridization of the Fe d and O p states, which is also responsible for its impressive electronic conductivity.