Structure analysis using X-ray and neutron powder diffraction and elemental mapping has been used to demonstrate that nominal A-site deficient Sr(2-x)FeMoO(6-δ) (0 ≤x≤ 0.5) compositions form as Mo-rich Sr(2)Fe(1-y)Mo(1+y)O(6) (0 ≤y≤ 0.2) perovskites at high temperatures and under reducing atmospheres. These materials show a gradual transition from the Fe and Mo rock salt ordered double perovskite structure to a B-site disordered arrangement. Analysis of the fractions of B-O-B' linkages revealed a gradual increase in the number of Mo-O-Mo linkages at the expense of the ferrimagnetic (FIM) Fe-O-Mo linkages that dominate the y = 0 material. All samples contain about 10-15% antiferromagnetic (AF) Fe-O-Fe linkages, independent of the degree of B-site ordering. The magnetic susceptibility of the y = 0.2 sample is characteristic of a small domain ferrimagnet (T(c)∼ 250 K), while room temperature neutron powder diffraction demonstrated the presence of G-type AF ordering linked to the Fe-O-Fe linkages (m(Fe) = 1.25(7)μ(B)). The high temperature thermoelectric properties are characteristic of a metal with a linear temperature dependence of the Seebeck coefficient, S (for all y) and electrical resistivity ρ (y≥ 0.1). The largest thermoelectric power factor S(2)/ρ = 0.12 mW m(-1) K(-1) is observed for Sr(2)FeMoO(6) at 1000 K.