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In the search for new spintronic materials with high spin-polarization at room-temperature, we have synthesized an osmium based double perovskite with a Curie-temperature of 725 K. Our combined experimental results confirm the existence of a sizable induced magnetic moment at the Os site, supported by band-structure calculations in agreement with a proposed kinetic energy driven mechanism of ferrimagnetism in these compounds. The intriguing property of Sr2CrOsO6 is that it is at the endpoint of a metal-insulator transition due to 5d band filling, and at the same time ferrimagnetism and high-spin polarization is preserved. PACS numbers: 61.12.Ld 75.50.Gg 75.50.Pp 75.50.Vv 81.05.ZxA so-called half-metal is a highly desired material for spintronics, as only charge carriers having one of the two possible polarization states contribute to conduction. In the class of the ferrimagnetic double perovskites such half-metals are well known, e. g. Sr 2 FeMoO 6 [1]. The here described compound Sr 2 CrOsO 6 is special, as it has a completely filled 5d t 2g minority spin orbital, while the majority spin channel is still gapped. It is thus at the endpoint of an ideally fully spin-polarized metal-insulator transition. At the metallic side of this transition we have the half-metallic materials Sr 2 CrWO 6 [2] and Sr 2 CrReO 6 [3,4]. Within the unique materials class of double perovskites, therefore, one can find high Curie-temperature ferrimagnets with spin-polarized conductivity ranging over several orders of magnitude from ferrimagnetic metallic to ferrimagnetic insulating tunable by electron doping. Note that Sr 2 CrOsO 6 , where a regular spin polarized 5d band is shifted below the Fermi level, is fundamentally different from a diluted magnetic semiconductor, where spin-polarized charge carriers derive from impurity states.While for simple perovskites as the half-metallic ferromagnetic manganites the Curie-temperature, T C , is in the highest case still close to room-temperature, halfmetallic ferrimagnetic double perovskites can have a considerably higher T C [5]. It has been suggested that ferrimagnetism in the double perovskites is kinetic energy driven [6,7,8]. In short, due to the hybridization of the exchange split 3d-orbitals of Fe 3+ (3d 5 , spin majority orbitals fully occupied) or Cr 3+ (3d 3 , only t 2g are fully occupied), and the non-magnetic 4d/5d-orbitals of * Electronic address: alff@oxide.tu-darmstadt.de Mo, W, Re or Os (N -sites), a kinetic energy gain is only possible for the minority spin carriers. This will lead to a corresponding shift of the bare energy levels at the non-magnetic site, and a strong tendency to half-metallic behavior. This mechanism is operative for the Fe 3+ and Cr 3+ (M sites) compounds [2], where all 3d majority spin states resp. all t 2g majority spin sates are fully occupied and represent localized spins. In agreement with band-structure calculations [1, 2,6,9,10,11] this mechanism is naturally associated with half-metallic behavior, as the spin-polarized conduction electrons mediate ...