The crystal and magnetic structures of double perovskites of the type Ba2-xSrxYOsO6 were studied by synchrotron X-ray and neutron powder diffraction methods, bulk magnetic susceptibility measurements, and X-ray absorption spectroscopy. The structures were refined using combined neutron and synchrotron data sets based on an ordered array of corner-sharing YO6 and OsO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fnover bar(3)m (x approximate to 0.6) -> I4/m (x approximate to 1.0) -> I2/m (x approximate to 1.6) -> P2(1)/n as the Sr content is increased, due to the introduction of cooperative tilting of the octahedra. Bulk magnetic susceptibility measurements demonstrate the oxides are all: anti-ferromagnets. The decrease in symmetry results in a nonlinear increase in the Neel temperature. Low-temperature :neutron diffraction measurements of selected examples show these to be type-I anti-ferromagnets. X-ray absorption spectra collected at the Os:L-3- and L-2-edges confirm the Os is pentavalent in all cases, and there is no detectable change in the covalency of the Os cation as the A-cation changes. Analysis of the L-3/L-2 branching ratio shows that the spin orbit coupling is constant and insignificant across the series.